The most poignant part of Donnie Darko comes when Donnie lies
down in his bed at the end of the movie, laughs, and breathes a sigh
of relief. In his talks with his psychologist throughout the movie,
he talks of his own lack of faith, the idea that everyone dies alone,
and other pessimistic outlooks on life. As he laughs and sighs, knowing
what is about to happen, it shows that he has accepted his death. Donnies
time traveling experience has helped him to find some sort
of faith. The first time the engine crashes he is not ready to die,
whereas the second time he is prepared and willing. The journey he has
taken, the experiences he has had with Frank, and the talks with his
psychologist have changed him enough that he is able to breathe a sigh
of relief and look forward to what is to come.
--Andy Carlson