There are many reasons to join a fandom, such as high interest in what it's about, your friends begged you to join, or a new way to find some friendly people. For me, it can easily be a label to someone who just wants to enjoy themselves. I had a bad experience with the Pokemon fandom, before Pokemon Go, my friends and I enjoyed the series greatly in high school. Up until Sophomore year where everyone made us the Weaboo's or whatever they called us. I'm pretty resilient when it comes to name calling or bullying since, well it doesn't really matter. BUT, the constant name calling influenced everyone in the school about us in a negative way. I tried to make new friends later on, but if they heard from other people about me or my friends before I got to them, they tried to poke fun at me about something I do in my past time. It was funny because I didn't even talk about it with people forcefully as if it were the best thing ever. In conclusion, fandoms are not for everyone. If the person really wants to devote their time to something that many find weird, good luck trying to find friends in high school (even though high school is the worst four years in most people's lives).
I don't think fandoms are bad because they help people to express enthusiasm for things they like (books, tv shows, music etc.) and they help people who may find it hard to make real life friends start to make friends within their fandom because they share a mutual interest. However, fandoms do have their share of unnecessary drama and stalkers.