I started years ago, I would have consistent mental breakdowns so I just started pouring boiling water on myself to get myself to stop thinking. It ended up scarring and people noticed so I freaked out and stopped, a couple years later shit happened and I started having panic attacks and mental breakdowns again so I began cutting. ]
Basically, it started out from mental breakdowns but because of how effective it is at numbing emotional pain (in exchange for physical pain) I continued without the panic attacks, it began feeling more like an obligation to me, I'd be in this rhythm where I'd cut every few days just to get that numb feeling all over again. Another part to it besides the obvious answers of numbing is I do it because I love to look at the scars. It's not some sexual thing, I just enjoy looking at everything I've done, it's great, it brings me immense satisfaction seeing what I've done to myself, contrary to a lot of people who are disgusted by them or don't care about them, I relish them. I just generally love looking at the cuts on my skin haha