Abu Hamid al-Ghazzali, 1058-1111, is sometimes called the "Thomas Aquinas" of Islam. He claimed for himself the privilege of ijtihad: the right to make fresh interpretations of the Qur'an, Hadith, and Islamic law instead of following traditional interpretations (taqlid). The so-called "gates of ijtihad " had been considered closed since the middle of the tenth century as Islamic theology and jurisprudence fell back on the authority of the traditions that had built up over the course of the first three hundred years of Islam.
From his Ihya 'Ulum al-Din (Revivification of the Sciences of Religion) (in John Alden Williams, The Word of Islam (Austin, Texas, 1994), pp. 159f.):
"'Love for God is the furthest reach of all stages, the sum of the highest degrees, and there is no stage after that of love except its fruit and its consequences, nor is there any stage before love which is not a prelude to it, such as penitence, long-suffering, and asceticism.
Yet some of the ulema deny the possibility of love for God and say that it means nothing more than persevering in obedience, while true love of God is impossible, except metaphorically. They also deny any intimacy with Him, or passionate longing for Him, or the delight of confiding in Him, and other consequences of his love. Thus we must of necessity deal with the matter here.
Whoever loves God for other than God's sake does so from ignorance, for among those of insight there is no true beloved save God Most High, and none deserving of love save Him. To explain this we shall turn to the five causes of love and show that all of them unite in God's truth.
The first cause of love is one's love for oneself, and one's own permanency, perfection, and continued existence, and one's hatred of perishing and nonexistence. This is the natural disposition of every living thing, and it cannot be imagined that anyone would deviate from it.
It necessarily tends to the deepest love of God, for one who knows oneself and knows one's Lord knows absolutely that one has no existence of oneself, and that self-existence, continued existence, and perfection of existence are all from God and to God and for God who is the Creator and Sustainer and Perfecter. If one loves oneself, then of necessity one must love God. If one does not, then it is because of ignorance of one's self and one's Lord, for love is the fruit of knowledge.
The second cause is that it is human nature to love one who bestows benefits and possessions on one, is kind of speech to one, and gives evil to those who give one evil. If one has true knowledge, one will know that the Benefactor is God alone, whose benefits to all His servants are beyond number...
The third cause is the love of benefactors for their own sake, even when the benefits do not reach you. This love is natural, for if information reached you of a king in a far country who was pious, just, wise, gentle with people, and kindly to them, and word reached you of another king who was cruel, arrogant, corrupt, shameless, and wicked, you would find in your heart an inclination to the first which was love, and a repugnance to the second which was hate. The first emotion is love of the benefactor simply because he is good, and not because he is good to you. This also necessitates the love of God; in fact, it necessitates loving none but God, for only He is truly good.
The fourth reason is to love every beautiful thing for its own beauty, and not for any satisfaction which can come from it...
The fifth cause is to love what is related and similar; for like inclines to like, as experience, report, and history all testify. This also necessitates loving God, because of inner similarity, which goes back not to resemblances of feature and form, but to inner significance, some of whose meanings we may mention in books, and some of which it is not permissible to write, but which must rather be left under a covering of dust until the travelers stumble upon them in the path when they have completed the conditions of the journey.'"