One of the most beautiful thing about the concept of worship in Islaam is the truly unique way in which it incorporates the feelings of love, fear and hope within the hearts of the worshippers of Allaah. Understanding how to combine these three qualities into the worship ('ibaadah) of Allaah is one of the most essential things that every Muslim must grasp, not least because every sect which has drifted from the Straight Path has strayed in this aspect of worship. The deviation of the other Religions in this regards is clear for all to see: "God is Love! Jesus is Love!" say the Christians, denying the fact that God should be feared too. As for the Jews, then their hearts are filled only with hope - hope in the belief that the Fire will not touch them since they are the "Promised People".
In Islaam however, no worship is complete without the presence of all three qualities: LOVE of Allaah, Hope in His Mercy and FEAR of His Punishment. Contemplate the opening Soorah of the Qur'aan - Sooratul Faatihah - and you will see for yourself.
Aaayah 1: "All Praise [and Thanks] are for Allaah, Rabb of all the worlds."
In this opening aayah (verse) of the Qur'aan, we have LOVE of Allaah and every time we recite this aayah we are testifying to our love for Allaah. How is that you ask? It is because in this aayah, we are acknowledging that Allaah is our Rabb and the Rabb of all the worlds. Rabb is usually translated as 'the Lord', but this translation does not do justification to this name of Allaah and all the meanings that this name carries. In reality Rabb means that Allaah is the Creator of everything; He sustains and nourishes everything; He gives life and death; anything good which we have is from Him; everything is dependent on Him and nothing can happen unless He wills it. Furthermore, for the Muslim He - the Rabb - is the One who has guided us to the TTruth and given us good morals and manners.
This is when we testify that Allaah is our Rabb, then we are acknowledging that He is the One who has given us so many blessings - so many that we tried to enumerate Hiis blessings upon us, we could not count them. So how could we not love Him? After all, we know that when someone gives us even a small gift or shows even a small act of kindness towards us, then we develop so much love for them on account of that - so imagine then the love we should  have for our Rabb who has given us everything: nice families, shelter, security, food, health, and above all, Guidance to Islaam and the Sunnah. We should be overwhelming in our love for Him and thank Him and say: "All Praises [and Thanks] are for Allaah, Rabbil-'aalameen."
Aayah 2: "The Rahman [Merciful] and the Raheem [Mercy-Giving]."
In the first aayah of Sooratul-Faatihah, Allaah mentioned that He was ar-Rabb. In the next aayah two more of His beautiful names are mentioned: ar-Rahmaan and ar-Raheem. Ar-Rahman means that He is the Most Merciful, that is, His Nature or His Essence is Merciful. Ar-Raheem means that He is the Giver of Mercy, in other words, His actions are full of Mercy and He shows Mercy to His creation. 
When we mention these two names of Allaah, then we have HOPE. Since He has described Himself as so full of Mercy then we have hope that He will forgive out sins, and no matter how numerous out sins may be, we should never lose this hope, because Allaah has told us: "O My slaves who have transgressed against themselves! Despair not of the Mercy of Allah verily Allah forgives all sins [except shirk]. Truly He is the Oft-Forgiving, the Mercy-Giving." [az-Zumar (39):53].
The need to have FEAR in our worship is shown by the next aayah of Sooratul-Faatihah:
Aayah 3: "King/Master of the Day of Judgment."
When we recite this aayah we are reminding ourselves of the Day of Judgment - that awful Day, when we shall all stand before Allaah, naked, uncircumcised and barefooted. Some will be sweating so much that it will go seventy arm-length into the earth. People will appear drunk though they are no. On this Day, every person will stand before Allaah and account for his sins, knowing that not even the smallest action which he did is hidden from Allaah:
"So whosoever does good equal to the weight of an atom shall see it. And whosoever does evil equal to the weight of an atom shall see it." [az-Zalzalah (99):7-8].
So when we recite this third aayah of Sooratul-Faatihah, we remind ourselves of this Judgment and accountability and that should bring about in us a sense of fear - that maybe out evil actions will be too great and we will be responsible for it - may Allaah protect us from such a fate.
Then the next verse goes on to say:
"You alone we worship." i.e. we single out Allah for our worship. And how do we worship Him? With LOVE, HOPE and FEAR. And in order to achieve these qualities, we need Allah's assistance, so we say then: "We seek Your Aid." [1:4].
STRIKING THE BALANCE
After understanding the need to have love, fear and hope in our worship, the question that now arises is: in what proportions should these qualities be present in our worship? Again we turn to the Qur'aan for our answer.
"Call upon Him with Fear and Hope." [al-A'raff(7):56].
"Their sides forsake their beds, to invoke their Lord in Fear and Hope." [as-Sajdah(32):16].
So both fear and hope should be present in our hearts in equal proportions. Anas reported that the Prophet , entered upon a young boy who was dying. The Prophet asked: "How are you?" The boy replied: "O Messenger of Allaah, I am in-between hoping in Allaah and fearing for my sins." The Prophet said: "The like of these two qualities do not unite in the heart of a servant except that Allaah gives him what he hopes for and protects him from what he feared." 
Therefore, whenever we do a good action, we should have hope that it has been accepted by Allaah, but at the same time we should also have fear that maybe it isn't enough or that the good deed has not been worthy of acceptance. Likewise when we sin, we should have hope that Allah will accept out Repentance and forgive us, but we should also fear that we may be accountable for it.
This balance should also be reflected in out da'wah (which of course is also worship). So when we invite others to the Truth, we should not give them the impression that they have "nothing to worry about"; nor do we suggest that they are doomed forever. Rather, we couple warnings with encouragement. We inform them of the horrors of the Fire as well as telling them about the bliss of Paradise. Just as we find Allaah telling us in the Qur'aan:
"Verily your Lord is Quick in Punishment and verily He is Oft-Forgiving, the Giver of Mercy." [al-A'raaf(7):167].
And our Righteous Predecessors used to say:
"He who worships Allaah with hope only is a Murji'ee. He who worships Him in fear only is a Harooree [Khaarijee]. And he who worships Allaah in love only is a Zindeeq [e.g. Soofees, Christians]. But he who worships Allaah in fear, love and hope is a Muwahhid Mu'min [a believer upon Tawheed." 
The Murji'ah were a sect that emerged within the first century of Islaam. The people of this sect (known as murji'ees) believe that sins do not affect faith, i.e. no matter what sins a person commits, his eemaan (faith) is complete and perfect. They worship Allaah only with hope because they believe that as along as one believed and testified to Islaam, they would enter Paradise regardless of their actions.
Unfortunately, all too often nowadays we find a somewhat similar attitude amongst the Muslims today, which is why we find that so often when we try to advise someone to turn to the Deen and abandon sin, they just point to their hearts and say, "Allaah knows what's in my heart", or that "Allah forgives" or something similar.
As for the Khawaarij, they worshipped Allaah in fear only because they held that anyone who committed major sins was a unbeliever and would therefore dwell in Hellfire forever. This, of course, is gross extremism and a straying from the correct Path. The Prophet said: "There were two men from Banoo Israa'eel who strove equally. One of them committed sins and the other strove hard in worship. And the one who strove in worship continued to see the other sin and kept saying to him: 'Desist'. So one day, he found him committing a sin and so said to him: 'Desist'. So he replied: "Leave me to my Lord; have you been sent as a watcher over me?" He said: "By Allaah, Allaah will nor forgive you, nor will Allaah admit you to Paradise." Then their soul were taken and they came together before the Lord of the Worlds. So He said to the one who strove in worship: "Did you have knowledge of Me, or did you have any power over what was in my Hands?" And He said to the sinner: "Go and enter Paradise through My Mercy." And He said to the other: "Take him to the Fire." Aboo Hurairah said: "By Him is Whose Hand is my soul! He spoke a word which destroyed this world and the Hereafter for him." 
Therefore we should never say of anyone that he or she is a 'lost cause' or 'doomed' or the like since this is a great sin. However, we fear for those who commit major sins because they have been threatened with punishment in the Qur'aan. It is up to Allaah whether He chooses to punish them or whether He will forgive them
The Zanaadiqah - or the heretics - are groups, such as many of the Soofees who have become like the Christians and reduced the religion to just 'love' and so all their talk and all their mystical practices are centered around developing more 'love'.
It is thus clear how imbalances in any of the three qualities of worship can lead to major deviation and straying from the True Guidance, Therefore it is important for every Muslim to inculcate all three things in his heart properly. As was mentioned before, the fear and hope should be in equal proportions  but as for love, then that should be uppermost, As Fudayl ibn Iyyaad (d.187H) said: "The love is better than fear. The fear checks us from sinning, and the love makes us to do what is prescribed with an open heart." 
May Allaah Subhaanahu wa Ta'aala grant us what
we hope for and protect us from what we fear. Aameen.
1. This is why the Prophet encouraged us to give gifts to one another. He said: "Give gifts to one another, you will come to love one another." [Hasan - Collected in al-Bukhaaree's al-Adabul-Mufrad & declared hasan by al-Albaanee in al-Irwaa(1601).
2. As explained by Ibnul-Qayyim (d.751H) in Badaa'i-ul-Faawaa'id & others.
3. Hasan - Collected by at-Tirmidhee & Ibn Maajah. Authenticated by al-Albaanee in Ahkaamul-Janaa'iz (no. 2).
4. Quoted by ibn Rajab in at-Takhweef minan-Naar as the saying of one of the Salaf.
5. Saheeh - Reported by Aboo Hurairah & collected in Aboo Daawood (eng. trans. vol.3 p.1365 no.4883). Authenticatd by al-Albaanee in Saheehul-Jaami (4455).
6. This was the opinion of most of the Salaf. Al-Fudayl ibn 'Iyyaad said that when one is healthy and well, then fear should predominate, but when terminally ill then hope should predominate - so that one should strive to do good when well and not despair of Allaah's Mercy when terminally ill. [At-Takhweef minan-Naar].
7. Reported in at-Takhweef minan-Naar
of Ibn Rajab. Note that some scholars hold that fear is better than love.
Allaah knows best.