The NFL is a gladiator sport, and some guys just can't walk away. Father Time comes calling for all of us sooner or later, and the truly gifted athletes usually can manage to keep from falling head-first into old age faster than other people.
But at some point, every athlete has to face the fact he isn't as young as he used to be and that his body just isn't going to carry him through another season at the level it used to.
Aches and pains become bigger and more debilitating, injuries come easier and take longer to heal, and the toll of being a professional football player just finally catches up and says payment is due in full.
The great athletes usually heed the call and try to bow out gracefully once it becomes apparent they've lost a step.
Other guys just ignore that voice in the back of their head telling them it's over and try and fight through it, reasoning their natural talent will get them through another year or two.
Here are a dozen NFL players who just couldn't let it go.