From a medical point of view, self-harming, or "cutting", releases endorphins. It can be a very powerful and addictive habit that is hard to break.

One important thing to note is that for many it hurts a lot less than you'd think, basically feeling like a light scratch.

Wikipedia has more detailed information and sources:

Those who engage in self-harm face the contradictory reality of harming themselves while at the same time obtaining relief from this act. It may even be hard for some to actually initiate cutting, but they often do because they know the relief that will follow. For some self-harmers this relief is primarily psychological while for others this feeling of relief comes from the beta endorphins released in the brain.

Endorphins are endogenous opioids that are released in response to physical injury, acting as natural painkillers and inducing pleasant feelings, and in response to self-harm would act to reduce tension and emotional distress. Many self-harmers report feeling very little to no pain while self-harming and, for some, deliberate self-harm may become a means of seeking pleasure.

https://en.wikipedia.org/wiki/Self-harm#Pathophysiology

To quote Trent Reznor, "I hurt myself today to see if I still feel."

I don't do "typical" self-harm. When my depression really gets me in it's grips, I be fine myself banging my head against the wall.

When the internal pain gets so bad, the external pain can be a distraction from it.

the reasoning varies from person to person but one thing it does do is make your body release dopamine which makes you feel good for a bit

A lot of reasons. Some include (but obviously are not limited to):

1. To distract from severe emotional pain

2. "Testing the waters" for suicide

3. Out of curiosity

4. To fit in with others

5. For attention

I myself have never done this, but I used to know people that did. Typically people who don't do it for attention try to hide it and generally do a pretty good job at keeping it hidden. People that make it obvious generally are doing it for attention, or they are at such a desperate time in their life that they are screaming for help.

Mental issues are incredibly varied and widely misunderstood. Every human brain is different and thats why it is very difficult for even health experts to explain why they happen, or even to come up with an exhaustive, definitive list of reasons.

For everyone it's different, for myself it started off as "I wonder what that would feel like". Obviously any mentally balanced person would never have that kind of thought occur, but it was curiosity that started it. Then I became more interested in the sensation and it almost started to become addictive.

So yes, I think the want to feel anything at all plays a big part for most people, and a lot do tend to find it addictive too.

I'm not sure you're going to find universal causes or justifications. It's been a few decades since I've fallen into those habits. For me, it was that my health really sucked and it was a way to exercise some minor control. I was in a fuckton of pain because I was sick and fuck me, right? But this pain right there? I chose that pain. That kind of stupidity. There's better ways to go about that than what I was doing.

For me a sharp pains snaps me out of a panic attack or spiral sometimes

No one has mentioned sensory overload, but it’s the main reason I see it.

Coping mechanics. It's a hard and fast distraction technique. It's grounding at its most base level. Feeling the skin rip stops some of the brains abilities to think.

Great weapon for keeping people alive.

Why not just Google the question

For clarity I don't self harm but I've fought the urge to several times....

For me it's often immense frustration. It seems to be that most people take out their frustrations on the people around them. Not everyone is frustrated enough to, and not everyone does it, but there's a nontrivial number of people who take it out on whomever they can, IMO, this is what leads to things like physical and emotional abuse but spouses, in some cases. IMO, bluntly, if you take out your frustrations on those you supposedly love, then you're in desperate need of either therapy, or a percussive rearrangement of your neurons....

For me, I silo things. If I'm frustrated at a situation, then that's what I take things out on, if I'm frustrated at a person, that's where that frustration is focused. Only when I'm in the midst of incredible stress/burnout do any of these lines ever get crossed and it signals to me that I need to take some time to rest and recover.

However, even frustrations are happening quickly and intensely, I get the urge to harm something. If my mind is properly siloing my feelings as I've tried to do, and I'm not frustrated to the point of uncontrollable (or nearly uncontrollable) anger at a person, instead being frustrated by a situation or circumstance or thing.... Then either I beat the crap out of an inanimate object to release that tension, or..... Well.... Take it out on myself.

There's a line in an obscure episode of the TV show scrubs, where doctor Cox says to JD that sometimes JD makes him so angry he's afraid he might hurt himself.... When I first saw that, I didn't comprehend what he meant... After being out in the world as a working adult for as long as I have been since first hearing that line, I get it. Sometimes the frustration needs to vent off, in some way or another, and sometimes the machine (me) that holds the pressure of that frustration starts to get damaged from holding the pressure.

It's fairly rare for me, I'm usually very calm and disciplined. Though, when it happens, it can be very intense.

Everyone's reasons are different, these are mine. I'm almost never in a situation where I feel like self harm is a good idea or something I am compelled to do. For me it's simply the failed containment of extreme emotion.

I use to do it because I was so frustrated that I felt it was the only thing that I could use to channel that it was s boiling hate to my self hard to express in words, at that time I was hoping with some bad decisions that ruined my life and they have consecuences still to this day. I was heavily depressed luckily I went for help and stopped doing it. Depression like that is like it never goes away just only gets better with the time.

I can answer for myself, it helps a lot when I'm stressed. When I'm stressed I tend to feel very shit about myself to begin with, and that coupled with the stress itself makes me do it to feel better. It works as well, although I wish it didn't.

Being unwell and hating themselves a lot.

It is a sad thing, but some people just don't feel like their worth the oxygen. Some do it as a "help me" sign, some genuinely want to just die, but it mostly comes down to loneliness, lack of self worth, and belief that people don't like you. In extreme cases, it's as a way to recive attention other than for help, as some people just want to be noticed at all.

I think others have covered most of it, but wanted to add that self-harm is often a coping mechanism for sexual assault.

Once, I've had thoughts of doing that. Never actually did, but I was in a bad enough state to want to, just to make a message of how my pain isn't something I can just 'suck up because others have had worse'. Kind of to say that my mental health was deteriorating so much that I would be willing to.