Well it used to happen when My parents would yell at me,or when things were stressful
in my life. But even when I have 0 stress I find myself doing it sometimes. Not all the time
But I will get the urge. Like if I am holding a knife I might get the urge to just slice my skin
with it. There's no one telling me to do it,I think my body just got addicting to the Rush
of endorphins that were released everytime I did it.