I think people like to feel special about themselves. When you were part of a small group of people who liked some band, and went to see them at small local venues for $5 a show you are "in the know". You are a part of that local music scene, and not one of the masses who just listens to what's on the radio. On one hand, you have discovered music that is good and not everyone listens to. You might say it is all about the music, but I think a big part of it is knowing that you "know" something that others don't. This makes you feel special about yourself. You aren't just a boring, uncritical person who listens to whatever commercial music is presented to them.
Then that band becomes slowly more popular, you are happy for their success until a certain point is reached. Now kids who you thought were just random lame kids with no taste in music at school are listening to them. If you want to see their show you have to buy tickets on ticketmaster, pay much more, and worry about sold out shows.
From a practical standpoint, it is now more difficult to see your favorite band. But I think the part that really bothers people is losing that feeling of being special. Everyone likes them now, there is nothing special about you liking them.
This same process can be applied to video games, films, craft beer, clothing/fashion trends, and probably any thing where you get very passionate fans.