Stress
If you were raised in the middle of a live volcano by a half-man, half-piranha and taught to defuse nuclear bombs while a horde of rabid spider monkeys poked you with sharp sticks, then the stress is nothing special. For everyone else, the NFL might be a little tense. Everybody expects you to be the very best, like no one ever was, every single minute of the day. Those kinds of expectations can stress a person out.
But hey, at least you're getting lots of exercise. That's supposed to be pretty good for stress...unless you get hurt and jeopardize your career. More on that in a minute.