>>718861323 (OP)
Evolution has a funny way of getting things done, so the system that regulates your perception of sustained pain works by pumping you with chemicals that just so happen to induce behavioural reinforcement. Endorphins. So if someone can get past the icky and rationally unwise part of cutting themselves (or any other painful activity), they will find themselves repeating it and any other behaviours proximal to it. External observers will typically describe this as an "addiction". The cutter will most likely describe it as "it feeling good".
You should not read too much into either of these descriptions. The names we call emotions don't correspond to any physical reality very well, because they primarily developed to serve social functions, as words usually do. The external observer calls it and "addiction" in order to disparage it, while the cutter says it "feels good" in order to retain the locus of control. Both terms describe "behaviour that is repeated" well enough to be used.
