cuting is done to punish yourself for not being "perfect" or to release the pain faster of bullies or something depressing that happened. boys do it to and no, people dont have issues, the people intruding in their happiness do.
I think it might have something to do with endorphins, the body's natural pain killers. They may get a feeling of euphoria. If they have problems they may use it to try and make themselves "feel better".
Everyone has a different reason. One might use physical pain as a focus area instead of emotional pain. One might do it for attention or to be noticed. One might do it to feel something different. There are many reasons.
I think the answers people have given have been very insightful and on-point. I just want to add that people who cut themselves can move beyond that. There are places you can get help from, just search suicide on google and although it's not suicide, they'll understand you and probably be able to guide you in how to get help. I'm so sorry you've had such difficult experiences in your young lives. Blessings on all of you, life can be a bitch.