Daisy married Tom Buchanan because he was rich and available and that was what beautiful debutantes did in Louisville in 1919.
She had been popular with the boys and men all her life. She had possibly even been infatuated and thought she was in love with Jay Gatsby, one of the army officers who came calling to her parents' house. But her parents had forbidden her to stay in contact with him, and so, after a brief period of time away from the social whirl, she returned to the society she thrived in.
After having been married to Tom for awhile, Daisy comes to understand the consequences of her marriage. She loves having access to Tom's money and the lifestyle it makes possible, but she is not happy in the marriage and sees no way out of her situation. Divorce was not an option in her position and at that time and place. She was trapped, paralyzed in the situation in which she found herself. The depth of her feelings were revealed as she told Nick about the birth of her child.
Tom was God knows where. I woke up out of the ether with an utterly abandoned feeling, and asked the nurse right away if it was a boy or a girl. She told me it was girl, and so I turned my head away and wept. 'All right,' I said, 'I'm glad it's a girl. And I hope she'll be a fool-that's the best thing a girl can be in this world, a beautiful little fool.'