Originally Posted by Fayrra

Well, generally people have these things called feelings. And then they feel stuff with them. If you're being mean, then it hurts them. Almost everyone has feelings. It's like a universal human trait.

Then there is this thing called rationality. If you're a generally rational and fair person, you're not going to want to hurt them. Whether you don't want to hurt them because you yourself wouldn't want to be hurt, or whether it's just because you're nice as heaven, is irrelevant. Either way, that's why people think it's wrong. And trust me, a lot of people on this earth think it's wrong.

Then there's a shit ton of other factors to take into account. Such as "What if that person was being mean as well?" or "What if being nice means lying?"