Daisy came from a very wealthy background, and she enjoyed all the pleasures and privileges that came with being young and beautiful and rich. At the time they met, however, Gatsby had nothing. He was a young Army officer stationed at the camp near her family mansion, one of many who came to visit and to fantasize about Daisy. At that time in his life, he had not yet built the dream character and the imaginary family history that he created for himself after the war.
Gatsby and Daisy did fall in passionate love, but he had to leave when the Army ordered him to do so, and Daisy didn't have the depth of commitment or strength of desire to wait for his return. When a wealthy suitor named Tom Buchanan appeared on the scene, Daisy aligned herself with the man who was immediately available.