I have only cut a couple of times. T and I talked about it A LOT because of me working in the mental health field I feel I "should know better", Here is what she told me

Many people cut (adults just hide it better) for a variety of reason
~ for some it is because the are frustrated and need to let that frustration out and cutting allows that to happen
~they are in a lot of pain an need to let it out
~for those who don't cry because of emotional pain they cut because it is okay to cry from physical pain.