Your brain knows that you are touching it. You send the signals saying “touch your armpit” and your armpit goes “okay, I’m gonna be touched.” When someone else touches you, your armpit doesn’t have that “warning” from your brain, it just says “IM BEING TOUCHED IM BEING TOUCHED AND YOU DIDNT WARN ME” hence the difference feeling. It’s basically because your nerves are already braced and expecting the feeling, so the sensation is less as severe.
Latest Answers