For someone so smart, I'm a little surprised it took this CTE study to push him over the edge. But maybe he wanted more conclusive data before leaving a job which paid him millions. :)
I would imagine being in front of a stadium full of fans and nationally televised each week would also be hard to give up. Not that people necessarily need the adoration, but it has to give you a high of some kind.
I can only imagine how much more difficult it is when you're a pro, the stadiums are bigger, and you are on TV.