I can see that with Witten. There's a line that Mark Schlerath said years ago on ESPN Radio that stuck with me: "I'm a competitor. Right now, you and I are playing a game that you don't even know exists, and I'm ahead by two points."
No doubt that Witten's a competitor too. But that same kind off no-off-switch fight in him that we love when he's shucking off Eagles defenders with no helmet on is the same kind of fight that won't give an inch to a rookie in training camp, even as a 50-year veteran. Hell, everyone praises Micharl Jordan's killer instinct too, but IMO that tied directly in to his pretty serious gambling issues, because everything became a game that he had to win. Competitiveness isn't always a positive thing.
Also: the whole "mentorship" thing is overblown in pro football. It's been mentioned with the Favre/Rodgers and Montana/Young examples earlier in this thread, but most veterans train rookies with the same amount of enthusiasm you'd have to train a new employee who'd been hired to take your job.