Gatsby tries to believe in the image he has created - of himself first, then of others. He has become wealthy through dubious means (bootlegging whiskey and gambling) but manages to keep up a clean "preppy" look just the same. He maintains his status by throwing lavish parties but does not seem to really care to much (on a personal level, that is) about the 'friends' he invites. He remains detached from the essential - developing true relationships with "real" people. He is a loner in the crowd:
Only Gatsby, who was relatively unselfish in his life, and whose primary flaw was a naive idealism, could be construed as fulfilling the author's vision of the American Dream. Throughout the novel are many references to his tendency to dream, but in fact, his world rests insecurely on a fairy's wing. On the flip side of the American Dream, then, is a naivete and a susceptibility to evil and poor-intentioned people.
- from http://www.enotes.com/great-gatsby/themes
Gatsby's fascination with Daisy is also on a superficial level. He accepts rather nonchalently her fickle nature as if it were an inherent part of her femininity. He thinks he can somehow buy her out through his money, at least for a while, and does not even lament the fact that they both reciprocally use each other. His willingness to "take the rap" for manslaughter to cover for Daisy reveals a certain naïve belief that he can beat the system one more time by just playing the role of 'the nice guy.' In the back of his mind, he might have even entertained the idea that this act would make Daisy indebted to him. Such lack of transparence unfortunately ends up costing him his life. He is murdered by revenge for a "crime" he did not commit.
I think that the answer to the question is much simpler yet more complicated than it first appears. Its greed.
I say this because it seems, even from a young age that Gatsby always wanted more.(confirmed by his father in a talk with Nick after his death and in many of the flashbacks) And while it seems that he had everything he still could not be content until he had more (Daisy). It seems that he personifies his wants of higher social standing and wealth in Daisy when he is in Louisville because she is the first person of the higher class where he strives to be that will associate with him and she becomes the object that he most covets. So in this regard it is not love that drives him to obsess over her, but rather his unsatisfiable greed.