I have been playing for 5 years now. One year I had 3 leagues . . . it became too much. I think having more than two leagues gets to be burdensome.
But FF actually makes football in general more enjoyable to me. Before I started playing I didn't follow other teams or the game in general nearly as closely as I do now. I am a Rams fan, and I rooted for the Rams. Most years that's hard, but I had a nice stretch there between 1999-2003. But when the Rams weren't on, football wasn't nearly as fun to watch unless it was a big game, or a game that tangentially impacted my team. I'm not the type of fan who naturally roots for any other team, but FF gives me a reason to be personally invested in almost every game that's on.
I like playing with people I know. I like researching, drafting, and trading. FF Draft day is one the top 5 most looked forward to events for me every year. I hate anonymous or public leagues where there is no personal connection to the other players, guys don't draft well, and almost never trade, and the crappy players lose interest halfway through the year and don't set their lineups.