Just doing his job...
He would certainly come back for the right money ... but he got to the point where he did not want to put his body through another season if he wasn't making a certain amount.
I am guessing that nobody has really gotten close to that amount. I personally think that he should not be retiring, he has good years left ... obviously not All-Pro or even Pro-Bowl seasons, but productive years left in him. I am kind of surprised he is ready to hang em up.
Favre-like skip camp tactic possibly???