It's a number of things. But I'm gonna get all Deep Symph on your a** and give you the most complex explanation LOL.
Think of humanity as an organism. What happens in one part of the body effects the rest of the body, one little vibration sends a shockwave through the whole thing. Think of you're self like a little ball of energy in this body, everything you do effects those around you, everything they do effects you.
When you see a hot guy you really like what happens? Do you just choose to walk over and act in just the perfect way that gets him to notice you and think you're awesome, only to then seem slightly disinterested and get him interested? That'd be ideal wouldn't it?? But what ACTUALLY happens... you freak out on the inside don't you. Your body reacts it starts going haywire and you feel this deep need to RUN!!!! But this other compulsion to APPROACH!!
Words are more than just syllables and sounds. They are the thoughts and feelings of one human being expressed to you. This knowledge that there is another human, which you're not sure if you're equal to or lesser than (even subconsciously) has now expressed that you are worthless. Has told you you embarrassed yourself. You don't see the face, but you know a person typed it. You don't know if it's a good looking awesome person you'd wish liked you, or some person you'd never talk to in a million years, but it's there, someone said it.
It took about 100 videos before I TRULY didn't care about hate. And I thought the same thing "these people are losers why do I care" Well when a stranger says "I LOVE YOUR VIDEOS!!!!" Do you think "Meh, it's a stranger, I don't even know if they mean it, so why should I care" Nope, you have just as much of a physical reaction to that, it's just the opposite, it's elation instead of fear.
Summation: Words kill, words bring life, you can use will power and the choice of facing your fears to overcome the pain they cause you and learn how to handle it when it happens, but unless you really lack in the empathy department, you WILL feel the words of others, both good and bad.