Idk how to explain it in any other way and again it depends on the situation. When you get severely depressed sometimes you just hurt so much that you feel empty and people do it because they want to feel something. When you hate yourself enough to hurt yourself that just kinda says for its self and their are other lots of other reasons but it also just relieves stress and emotional pain

You cut because you have emotional pain for me, just screwed up family and mother, yes cutting is like a drug I got addicted but it doesn't really hurt it feels peaceful now I usually cut myself with my fingernails but I'm getting better I'm very empty and severely depressed but alcohol and stuff doesn't help I attempted suicide twice now I might just jump in front of a car I give up life please don't follow my life though

Coming from a person that suffers from a MDD disorder; I really don't want to encourage anything of this sort.

Seek professional help.