I cant answer that with a definite answer. But some theories (cant u tell im a psychology student Mrs. Docdragon lol)
One theory is that people dont know about it and they think we are going to kill ourselves or something
Another is that people cant comprehend why we would cut, burn, bruse our own bodys for releif.
Another one is that its a slow way of killing ourselfs which we all know its not true...

These are some reasons why others think its so bad to cut.
I cant tell you what u see when u cut for me its a way to release all the pain inside and have peace even if its for a second. And that i learned that talking, crying, venting etc is wrong and shamefull and i turned to cutting as the only acceptable way to vent and also punish myself for not doing something right or doing something wrong also. an example would be if i didnt figure out what my mom means and did something like oh say she wanted me to wash the dog do the dishes, vaccum, and clean the fish tank and i did all but wash the dog. I would take forgetting to wash the dog as a failer and i would punish myself. OOOh sorry this is a long post. But i hope this helps.