Why exactly do people self harm? Is it different for everyone?

I know that different people like self harm for different reasons—i suppose what turns us on to it— for a couple of my current and past friends its about feeling pain or seeing blood. For me it's both but more often seeing blood.

My question isn't this. My question is why us humans decide to go "yeah I want to self harm." Why do humans do this?

It's a little hard to articulate though the question is why do we as humans decide to self harm deeper then just feeling pain or seeing blood?

Sorry if im still wording it incorrectly.