I have followed the Bills since their inception. I still watch all their games and follow them as a fan. I used to watch all other possible games (1:00, 4:00, 8:00, Monday night but no more.
Maturity, more time with family, other interests... I could care less about other teams. I will generally watch the playoffs and Super Bowl but a lot of times only a half or so.
Why... spoiled athletes, other interests... I don't have a great explanation, maybe when you get in your sixties you find there is more to life than the NFL.
I have had a similar experience with all major professional sports. I lived for the NBA (old Celtic/Lakers rivalries, then Magic versus Bird, but then team play went away and I cant remember the last time I watched an NBA game.