There's another train of thought that was brought up when Favre kept going back forth with retirement and that is: once a player retires from football their entire way of life changes, the structure that they've have from age of 10+ is completely gone, training, goals, pressure to be the best, fame, teammate comaraderie, etc, essentially not knowing what to do with themselves or any goals. For some players, certainly not all it is obvious that while going through school their physical ability was focused on while their mental aspect was neglected (Look at some players Wonderlic scores). Their inability to cope with this change can lead to depression.
Also, like knightgames showed with his nephew, just one hit can cause quite a bit of damage. I got a concussion from football around that same age. For months afterward I would forget mid conversation, even mid sentence what was happening. Turned out that I was having milli-second seizures that "reset" my brain... There wasn't really any cure so I had to wait it out, now about 13 years later I don't really have the problem anymore, although I still don't have the greatest memory.
Now imagine these NFL players whose "career" lasts 10-15 years, not including another 4 in high school and another 2+ in college. Those hits can certainly add up.