he was so selfish

I agree that it's easy to say that, but no one knows how they would react until they are faced with a near death experience. As the plane is cruising, he is so nervous, and fearful, but once he is faced that his death is probable, it changes his entire outlook. He figures if he is going to die, then so be it, and finds peace with the idea. Unlike the others, something flips in his brain, and makes him absolutely ready to die. After the crash, when he willingly eats the strawberries, and they have no effect, it only reinforced his fearless state of mind. Just like the Nam vets they talk about in the movie, he felt like he was invincible. He became addicted the fearless feeling, and showed it when he stood on the building top to regain the "rush". Once he completes his mission to help the woman who lost her 2 year old feel alive again, he feels deflated, and wishes he felt alive again as well. Then, at the end, just before asking his wife to save him, he eats the strawberry, and has the reaction. He was back to his old self. He was alive once more.
Have you ever done a heavy dose of psychedelics? They can have a similar effect on a person. It is pure ego death. When you return to reality, you are changed forever. They say when you're close to death, the body releases a large amount of it's own psychedelic (from the pineal gland in the brain, aka the 3rd eye). This substance, known as DMT is the most powerful psychedelic on the planet, and can have profound effects on a person. So, it's not far fetched to completely change one's state of mind on the cusp of a dramatic death.

He had PTSD. Clearly he was in a state of manic psychosis and I would think that was so clearly obvious. You missed the whole point of the film. And I thought you were sharp.