I honestly believe Belichick has some strange variant of autism. Does that sound strange? I know he likes to fish, has a nice gal to keep him company, etc., but picturing him retired is even creepier than imagining him in that hoodie looking through my garbage. What else does he do with his life but coach football? I bet that even when he's out on his boat, at least part of his mind is somewhere else, coaching football.
It's sort of perfect to discuss this on this board, because the analogy to music is so apt. I'm old, and not at risk to do what is necessary to gain an audience as an indie rock musician. However, there is no way I will ever stop making it in my spare bedroom. Because that's who I am; at an early point in life I fell in love with music and now it's far, far too late to undo that.
That's him and football.
For a long time I didn't think that his advantage was about commitment -- if you've ever known pro (or even top-flight Div I) athletes, they're all hugely committed. But there's committing, and then there's committing, and that's what he gets. It really is astonishing. An example: the "no big plays" defenses the Pats have rolled with the past decade. It will drive a bystander nuts. Too many first downs given up! Too much time of possession for the opponent! And as we've especially seen when the opposing offense isn't prone to mistakes, it doesn't always work.
This is why the Pats have so many crucial goal-line defensive plays. They keep shrinking the field until the math (less area to cover) favors them and take their chances. It's mathematically brilliant.
The counter-argument is also frequently viewable: athletes get emotional, over-extend, and ... they lose the battle and give up a big play.
It takes a lot of guts (and other body parts) to stand your ground in an NFL playoff game; Belichick's genius is how he gets guys to do this without question and continuously. Should've been a general.