Some where in the vicinity of the summer of 1996, Brett Favre became addicted to vicodin. Addicted to such a serious degree, that he was in a rehabilitation center for 46 days. He has since beat his addiction, and has became known as the "Iron Man."
From here on out is my own speculation.
Somewhere during the 1996 regular season, he realized that the pain he fought through during training camp, practice, and his regular Joe life in Mississippi pruning trees and bushes, went away during games. The adrenaline rush was amazing, intoxicating, and his perfect antidote.
He no longer needed his pain medications as he was in tip-top shape. But he missed all the feelings and sensations that came with pain killers. He found an incredible inflammation of his nerves, and suddenly, he felt invincible.
And he can't let that go. He can't leave his mental paradise, the spotlight, competition, and the adornment of all his fans. It will hurt him more than he knows, but his window is a hair-shy of being shut.
Brad Childress has shut the door on signing Favre. Soon he will face the withdrawl that comes with the off-season, every single season, until the day he dies.