Commenting to complain vs just ignoring the post

I don't understand why people comment on topics just to complain about it's contents or because similar topics have been talked about a lot already. Why not just ignore it and carry on instead of wasting time out of your day to comment on something you don't like??
I'm actually curious, I see this often.
I'm honestly curious haha 
I won't lie, I've done it but unless I have something to contribute to the post even if I don't like it or it's annoying, I just ignore the topic(s) or post.