Reason:-
Wearing headphone is not bad; but wearing it for more than 30 minutes at a go is bad for the ears. When you put on headphones, you cover your ears from the natural air, which increases production of bacteria by 700% in 1hr. Your ear drum naturally can not bear influx of bacteria in that quantity so it starts to react by paining or hearing loss in long term.
Secondly, when you are listening to fast music/ high beat sound which goes direct to your brain due to headphones, it affects the fine tissue/membrane of the brain eventually listener becomes hyper or excited which is bad in long run.