We learn from the very beginning that Ada Taylor is The Girl to Cullen (and to a lot of other boys in Lily, Arkansas). The hottest girl in school, she has this mysterious, dangerous air about her. All her boyfriends end up dead, though somehow this doesn't deter any of her admirers or suitors. As Cullen expresses, though, she's worth it:
After that, it almost seemed like a ridiculous thing to date Ada Taylor, or even go near her. But that didn't matter much to the young men of Lily, even me. The unspoken philosophy of all those in love with Ada was something like this: If I have to die to get that, then death it is. (1.16)
Cullen is so determined to be with Ada that he does so despite all the warnings and signs that it might not be a good idea. Even when Lucas warns him that Ada might just be dating him because she feels sorry for him, Cullen is unfazed; he doesn't care if that's the case, and he'll totally take her pity.
In the end, Ada Taylor leaves Cullen for her paralyzed ex-boyfriend, Russell Quitman, and it becomes clear that he never really knew her anyway. He was just in love with an ideal. His adoration is based on empty premises, which is interesting in a book that keeps bumping up against religion.