I don't remember when exactly I got into it, but it was a big thing for me in highschool. I'd be sweating my fat ass off every day because I walked to school in Florida heat wearing pants and a heavy jacket to hide all the places I'd marked myself up. My hoodies, shirts, jackets and sheets were all stained with blood, and when I wore a long-sleeve shirt after freshly slicing myself up, it'd stick to my arm. Mostly used a rectangular razor blade (as opposed to the angled type you'd see in a utility knife/boxcutter)... and I'm really not sure what to make of it. It's pretty common to hear people say that cutters just want to feel like they have control over their bodies and lives, and I'd say that's pretty close to how it got me off... aside from the endorphins triggered by pain and what-have-you, because the effects are both physiological and psychological.
I mean, all I can really say is that I was seriously messed up, and then it gradually went away for no discernible reason. It sure as hell wasn't a cry for attention since all I really wanted back then was to be loved, and nobody's going to give the time of day to someone that hates themselves enough to make their arms and legs look like patchwork. If I had to identify a culprit, I'd probably blame the cocktail of pharmaceuticals I was being grossly over-prescribed back in the day; I went from being a quiet, shy, timid kid to a cracked-out, paranoid, manic-depressive train wreck that couldn't keep his fucking mouth shut to save his life, and I was too high on massive amounts of Adderall and Prozac to think they were doing anything but helping. That stuff fucked me up, and it became my only source of good feelings in a life I was thoroughly sick of living. It was like living as a junkie, except the drugs didn't give me anything to live for. I haven't touched prescription anything in over four years (the only exception being the stuff prescribed by physicians, fuck psychiatrists), and now I've been kind of having to slingshot my way into adulthood by making up for lost time and responsibility (college is hard as FUCK if you spent your highschool years as a fuck-up instead of a student).
I've put it behind me entirely, though. The only time I'll cut myself on purpose is if I want to test the sharpness of a newly-sharpened knife (and even then, not enough to split the skin), and whenever I need an outlet, I'll just go exercise (which I intend to do right after I post this) or get together with friends and party or chill or whatever. At least the whole nightmare wasn't for nothing: it gave me a strong sense of empathy, made me appreciate how good life can be, and more-or-less removed my fear of death. Sure as fuck wouldn't do it again, though.