The Butterfly Effect

Even with multiple trips into his past, Evan (Ashton Kutcher) is unable to "create" a present in which he and Kayleigh (Amy Smart) can be together and happy. He finally realizes that to truly show his love for Kayleigh, he has to let her go. He goes back to their first meeting and offends her so much that she despises him. Then, when her parents divorce, she leaves town with her mother, rather than stay with her molesting father just to be close to Evan. Back in the present, everyone ends up successful and happy, but Evan and Kayleigh don't know each other.
Thanks, Lisa and Charles!

Several folks have written in to let us know that the DVD has a different, "Director's Cut" ending: Evan realizes that his multiple trips have made everyone miserable. He finds old home movies and watches the film of his own birth. He uses this to travel even further back into his past, and kills himself by wrapping the umbilical cord around his neck. In a final present-day view, we see that his mother has had another child later, and that she, Kayleigh, and all of his friends are happy.