My reason for self harm is similar to what's already been said, and I think there are loads of reasons why someone would.
I started self harming when I was 14, I felt so much emotional pain I felt like I had to act on it to do something to lessen the pain, definitely an impulsive act. I still self harm now and I'm 23, ive always hated the pain but when I'm really distressed self harming almost stops that feeling for abit, or I'll do it when i don't feel real anymore and it reminds me I'm a human (if that makes sense), it becomes a habit that's really hard to stop.
I get strong impulses now to self harm whenever I get in a distressed state, I wish I didn't self harm, I've not worn a t shirt in public or Infront of my family since I started at 14, I've been in hospital with infections, and I've spent way to much time seeing dr's and nurses or trying to persuade them I don't need treatment, and I hate the fact that I self harm but I still can't stop.
I wish I never started in the first place but I feel like I'd be dead already if I didn't get the weird relief that if gives me.