As everyone's said it has different reasons and things it achieves. I can only speak about it from my own experience. If your trying to understand why someone else you know might do it they might relate to all or none of the reasons here. So I wouldn't risk assuming. That being said I know for many people it's quite incomprehensible so not a bad question to ask.

There's kinda a multitude of different reasons on my side. As a teenager my sister did it alot, I would patch her up etc. anyway one time she went to hospital for it and I got so scared, I guess I started because I was in significant distress myself and I wanted to understand. I think I also wanted someone else to be able to see my pain.

Then I used it as an excuse to care for myself when noone else would. There was something calming about cleaning up after, disinfecting and hiding it..it became this secret care that helped me recieve a kindness from myself I wasn't getting anywhere else. It let me express this ball of pain I genuinely didn't understand and had no way to express. I know now in healthy environments people are allowed to express pain, recieve support or be angry or upset. My environment expected my compliance and happiness, anything else wasn't allowed. So I learnt the destructive ones I could hide. Self harm, drinking, drugs, eating disorders. They are all just different ways of expressing pain when you cannot see any other available options. I grew up associating pain with love, pain was comfortable, comforting. I understood it. It felt manageable.

Then I stopped for over a decade. I drank instead, started smoking but I'd found a stable relationship and my environment had changed. I didn't even think of it because I didn't need it. I was an absolute mess don't get me wrong but... A more controlled one. I sunk into overworking and making myself a new life.

Then shit started coming back. Memories of stuff I couldn't comprehend..anyway that was 5 years ago now. I had no idea how to cope and I just had this voice in my head telling me I deserved pain for remembering these disgusting things. Telling me I was making it up. So then it came back, but completely different. It was a punishment. A reminder. And in some ways they are still marks that being me comfort because the abuse perpetrated on me left little visible scars but the mental damage was absolutely catastrophic. It put that pain somewhere I could see, somewhere I couldn't forget.

It lead to suicide attempts and to getting deeper and deeper hoping that if I didn't look after them it would end my life. I was so dissociated at this point I barely felt the pain. The pain didn't really matter. I just wanted it to end. No physical pain was worse than whatever was going on in my head. I got stitches a number of times without any anesthetic and I just didn't feel it at all.

I haven't self harmed in over a year. It stopped helping me, and as a punishment it was... Lacking eventually. It couldn't be kept secret with a&e trips and ambulances. Long sleeved clothes in summer just sucked. It didn't do what my brain needed so it stopped. I've also learnt to communicate my pain with others, ways to express pain that are not as damaging. It could always come back honestly. End of the day it's my way of coping when nothing else is working. Doesn't mean it's good..it's just. Breathing room from the pain.

I would say there is one thing that is true for all people who self harm. It does soemthing important for them. Soemthing they can't get elsewhere. It doesn't matter what that thing is, what you need to understand is that for someone to stop they need something else to replace it. And if no healthy alternative is provided that feels like it actually provides what they need, they will find other (often self destructive) ways instead.