The thing is, that "normal" people who enjoy something, they will enjoy it and then move on with their lifes. Even if they want to discuss or share their recent experience with other people who had the same experience, they will do it for few days and move on.
At the end, any "normal" person values real life more than fantasy worlds that we experience by reading books, watching anime/movies/shows or playing games.
Therefore "fandoms" are full with people who are obsessed with things they enjoy. These people stick around all the time, they can't stop thinking about it, because their life sucks and they can't deal with it.
Being obsessed with something is obviously not healthy and these people are mostly disturbed.