The Four Loves by C. S. Lewis

C.S. Lewis's famous work on the nature of love divides love into four categories: Affection, Friendship, Eros and Charity. The first three are loves which come naturally to the human race.

Charity, however, the Gift-love of God, is divine in its source and expression, and without the sweetening grace of this supernatural love, the natural loves become distorted and even dangerous