While I wouldn't stop liking a band simply because they're "too popular", I would like to highlight that:
1) Bands that become popular may well have done so via watering down their music, making it largely homogeneous with whatever else is selling well.
2) A hugely important aspect of a band is its fan community. If a band's fan community is made up of peoples whose taste in music is utterly soul-less, then it taints the music sometimes.
3) Overplay. Probably the main thing that even excellent bands suffer from. Hear a great song on the radio a few times, and it's great. Hear it 5-10 times a day every day and you
will end up hating it. I regularly get annoyed whenever I hear anything off the new Muse album any more as one example. Plus it's clearly inferior to the previous two, that doesn't help. I just hope they don't suffer number 4...
4) Critics do not know how to recognise the good from the bad when it comes to popular bands. They stop judging the music on its integrity and genuine greatness, but instead on the popular movement it follows. Thus good bands may put out a bad album, and still get rave reviews, giving them reason to continue on the slope that album brought them on.
5) Ego for no reason. 'Nuff said.