It's hard to feel sorry for them because a) it's voluntary, b) they make more money in their 3-4 year career than most people make in their lives, c) when their football career is over the can retire early or start a second career if they like, and d) there are a ton of fringe benefits.
Despite the physical toll, I suspect the vast majority of American men would sign up for an NFL career in a heartbeat if they could.