Donnie is mentally ill, lets say some form of depression. He decides to take his own life; an overdose before he goes to bed. As he lays there, he justifies his actions by coming up with the story that we watch.

I've almost been in that position myself. Everyone is saying "things will get better", and "something might happen tomorrow which could change your life entirely", and "if you kill yourself, it will harm your family".
So, he imagines a scenario in which even if he met the girl of his dreams, she and his family would still be better of without him... well, they would be alive. He justifies taking his own life to save others.
The only bit that we see for real is towards the end when he is in bed, laughing quietly to himself. From this perspective, it's a very empty, sad film. But the saddest thing is that somewhere, right now, someone is seeing their own version of it, just as they breathe their last breath."

Posted by: :-) mark on Aug 16, 05 Source: Metaphilm.com