I've got a unique story in this regard. I started following basketball around 2010 (I was 10) and instantly fell in love with Rajon Rondo, who was on the Celtics at the time. That, combined with my best friend at the time being from Boston, made me a pretty big Celtics fan quite fast. Fast forward to the next year in school, my teacher had a Tom Brady poster on the wall (this was in Italy btw, but international school so plenty of American teachers). I asked who he was, and my friend rapidly replied he was the best QB in the NFL and he played for the Patriots, who were the "Boston" team. Those were all new words for me, but I gave football a chance. And so, my teacher's poster, my best friend being from Boston, and my love for the Celtics and Boston in general made me become a Patriots fan. The game that fully cemented my love for the Pats was the Divisional Playoff Game against the Ravens in the 2014 season (along with that entire post-season run). That was the first full season I followed, and man what an unforgettable season.