When we tickle ourselves, our mind knows in advance that is supposed to happen. So we develop some sort of a defence to counter the effect of the tickle.\nWhen somebody else tickles us suddenly our body is not prepared for that change of state.\n\n Even if u hit urself (not too hard) it pains lesser than when some body else hits u suddenly.
Why do we get tickled when somebody tickles us and we don't when we tickle ourselves?