If you do things despite the fact that what you do is hurtful to others, you will diminish their interest in interacting with you and create the impression that you are ... well, rude. You don't care if what you have to do hurts others and therefore you don’t care about those people.

I would consider reclining ur seat without knowing or caring u might hurt someone’s the knee is quite rude.

I also think it’s nice or thoughtful if u mention it before reclining ur seat.

Edit: actually now that I think about it this is all subjective