I wasn't a real football fan as a kid. When I was about 10 (1981), I liked the Steelers. Back then ,where I live, most kids either liked the Steelers or the Cowboys.
When I got to high school, my best friend was a big football fan and his team was the Broncos. I loosely latched onto the Browns, just because they were rivals and I was playfully going against my buddy.
When the Bills played the Giants in the Super Bowl, I was in the Navy. I was at a shipmate's house to watch the game. I figured that was the best time to finally pick a team, since both teams were NY teams (kind of). I told my friends that I was flipping a quarter - heads, I root for the Bills; tails, I root for the Giants. And whichever team I ended up with, I'd stay with for the rest of my life.
It landed heads. And here I am decades later.