Nathan Haller
That's the point. You don't need to know why he did it, you just need to know that he did and he cares for her. That's what Hollywood has done. We expect every answer to be given to us, but the mystery is (most of the time) the best part. Watch Ridley Scott's Blade Runner The Final Cut (please do not watch the US version) or listen to Yoda's monologue about what the Force is in Empire Strikes Back. There's mystery in those films unlike nowadays.