system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people cut themselves?

I'm not trying to trivialize depression; im just curious what the psychology behind it is. I can't imagine why I would inflict harm upon myself to cope with problems. If anything, why don't people turn to, for example, alcohol instead?

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Edit: Just wanted to say thanks to everyone who took the time to give me an explanation, whether it be through their own experiences, the experiences of others, or just things they've researched; all of them have been interesting and helpful.

So, from what I've gathered from the comments, self-harm is the result of a variety of things: disorders/depression, stress, general unhappiness, a desire for penance, low self-esteem, etc. In some cases, it's a way to feel in control of at least one aspect of life; in others, it's to feel some sort of happiness through an endorphin rush. Some do it as an act of self-destruction with low self-esteem being the catalyst; others do it to keep themselves from doing worse to themselves, i.e. suicide.

As for choosing self-harm over substance abuse, cutting is free, easy to do, and doesn't (temporarily) mentally impair those who are looking to regain control over some part of their lives.

I hope I didn't misinterpret any of the comments when compiling them; please correct me if I'm wrong on any of this, and again, I really appreciate the answers. Thanks!
gpt
A feature of depression is feeling numb and unable to experience pleasure. Some people describe cutting as allowing them to "feel something", it can produce positive chemical signals in a similar fashion to exercise (endorphins, as another comment put it). 

Other factors include elements of self-hatred which can be seen in depression as well as a syndrome called borderline personality disorder which is strongly associated with self-harm. 

Self harm can sometimes also be used to try and communicate with others or alter their behaviour, for instance to try and show how much emotional pain the person is in.