She ran away from her coming groom and husband to travel with the Doctor (S5E01) the day before the wedding. Arriving at the same night after several adventures, she has made her decision and would rather have the Doctor instead of Rory.
Also, the Doctor is someone she had pretty much fantasized about since she was a little girl and was always a perfect guy in her mind. When he returns to her on the night before her wedding, she is very anxious about returning the next morning for her wedding. After the experiences they had together, combined with her childhood fantasies, she wants him.