Why did James Franco cut his arm off for that movie?

I just watched that 127 Hours movie with James Franco, and in case you haven't seen it, he has to cut his arm off!!! Like from the center of his forearm.. For a movie!!! They couldn't pay me enough to do that!! I just don't feel like it would be worth it all to cut your arm off, just for a movie. Not to mention, he had to crush it under a rock for the entire filming process!!! I would never actually hurt myself like that, no matter how much they paid me. It wasn't even that great of a movie anyways!!! It was decent, but it wasn't really all that successful.. I just can't get over this. And I understand he wanted to commit to the part, but I really think he's destroyed his career, and really messed up his life with that one stupid move.. So, just why would he do this?? For the money? Cause I don't think anyone could pay me to cut my arm off. ):