Usually people pick the favorite team that was good when they started watching, or the team with the favorite player
For example there are many Lakers and Celtics fans because the NBA started to be broadcast on TV during the 80s, the second boom was at the end of the 90s/early 00s so even more Lakers fans, Spurs fans, Pistons fans, etc.
I support or sympathize all teams in Los Angeles, so Lakers, Sparks, UCLA, Dodgers, Ducks, and Kings, for NFL Buccaneers because fuck my rules, my cousin does the same with teams from Minnesota