in order to really answer the question of why one is seen as acceptable and the other not i would like to take you as an example.
how do you feel before you self harm?
how do you feel during?
how do you feel after self harm?
i want you to include both mental and physical effects. you don't have to 'know' to answer, if you notice that you usually have an argument with your parents before you harm yourself include that, even if you don't connect it directly to self harming.