A good question a lot of folks ask..I honestly believe that it's because parents are not taught/allowed to discipline properly at the earliest ages. The habit of associating pain with anger seems common among those whose parents allow the child to 'direct themselves', to learn by trial and error. The habit may also begin from an accidental scratch to the cheek by a parent or any other child when angry, giving the signal to the child that this is what they should do to themselves when angry.
I am a Christian, living by Christian/Judeo values taught through Holy Scriptures, which are firm about not letting a child grow without clear, distinct boundaries, family rules, appropriate disciple ["spare the rod of disciple and you will spoil the child."] leading to a spoiled child which will only get worse, never better. Child psychology has failed society in many ways by letting children lash out, express inappropriately at any time/ any place, which teaches the wrong messages, gives the child rage...in my opinion. My prayer for you is that you will come to the place where you need the Highest knowledge, wisdom, understanding, patience, and forgiveness for not knowing everything...crying out to Him to show you who He is/ has done/promises to do for all who will simply trust Him with their very life as I have done for twenty five years now without regret or wavering! God bless and help you. : )