The first time Jay Gatsby met Daisy, she was eighteen-year-old Daisy Fay, a beautiful and well-to-do young woman who lived with her parents in a large home close to Camp Taylor, a training and staging point for troops being prepared to depart for the European front. As was the case with many of the officers stationed at Camp Taylor, Gatsby was attracted to this attractive Southern belle and joined the others who "demanded the privilege of monopolizing her that night."
Daisy had material wealth, beauty, refinement, charm - all the qualities Gatsby felt he had not acquired from his parents but was determined to obtain. He became infatuated with Daisy for what she had as well as for who she was; Daisy returned the infatuation and was heartbroken (at least for a few months) when he was shipped overseas.
At first, it really seems to have been Daisy's incredible wealth and status that attracted Gatsby all those years ago. Nick says that
"Gatsby was aware of the youth and mystery that wealth imprisons and preserves, of the freshness of many clothes, and of Daisy, gleaming like silver, safe and proud above the hot struggles of the poor."
She was beautiful and desired and rich, and she represents everything that he seems to have been striving to attain throughout his life. He wanted the security that she felt, the coolness that she seemed to embody without even trying. He lied to her, however, when he led her to believe that he was from the same kind of family and background as she was.
Then, though, he fell in love with her. Nick says that
"[Gatsby] had intended, probably, to take what he could and go -- but now he found that he had committed himself to the following of a grail."
Gatsby likely didn't intend to spend his life with Miss Daisy Fay, the most popular girl in Louisville, but he soon came to find that he needed her. And then he was sent away to war.