For his family.
And why wouldn't that be the truth? He's never expressed a desire to be anything more than a simple, family man who makes a living playing professional football. The obvious contradiction with him being the manner in which he approached the game - The legendary offseason workout regimen - His ferocity on the field juxtaposed so perfectly against his quiet demeanor away from it. Never again will there be another Troy Polamalu in the National Football League. And as sad as it may be to realize he's retired from the game, it's doubly satisfying to have witnessed his career and have an opportunity to reflect upon it.
He is the best I've seen play the game. Full stop.
His career arc is old hat: Young man plays football with such reckless abandon and the inevitable injuries only serve to hasten the aging process. With Polamalu we always knew this was how it would go. How could it not? He spent more time in the trenches with men 100lbs heavier than him, looking for opportunities amid the scrum, than any other safety in the game.
But it was in the searching for opportunities that he set himself apart. Great players tend to take advantage of opportunities when they present themselves, but Polamalu understood how to force opportunities at the most seemingly inopportune moments. His coaches and teammates entrusted him with almost unprecedented autonomy - a ghost within a play chart full of Xs and Os - but he almost never abused the privilege. His value as a player lain in his acumen for the game as much as in any of his physical abilities.
We often called him a risk taker, but the truth lies somewhere between that and genius. Polamalu calculated risk into every decision on and off the field and, even more than that, when he took a risk, he committed fully. As a result, he made some of the most memorable plays you'll ever see. He caught his opponents off-guard even when they knew he was coming by doing things they literally didn't think were possible:
Leaping lines of scrimmage to break up plays that had barely started.
Intercepting uncatchable passes.
Tackling faster running backs in the backfield (see Johnson, Chris).
Gaining leverage on 400lb offensive linemen.
Those are the things I'll remember about Troy Polamalu on the football field. Much has been and will be said about him as a player, leader and role-model now that hindsight has become all we have. He's one of those rare examples where nothing negative should (or could) be said about any of the three.
