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Proprioception.

Your mind knows where your hand is at all times and will tense up against the pressure you apply.

When someone else massages you, your body does not know exactly where there hands are and so will keep the muscles relaxed.

I assume this is related to the rain you can't tickle yourself. Thanks for an actual word to refer to about it.

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Also: great word.

Personally, I find very difficult to massage myself, let’s say I want to relax my legs: Mi have to stiffen my arm to apply the pressure, then my chest and abdomen and push onto the leg. It’s very difficult to let the leg relax while the rest of the body is stiffening. If you don’t relax the part of the body, the massage won’t penetrate the first centimeter of the body. Muscles must be relaxed in order to let the pressure go deep.

Plus all the right things o saw in the other comments.

Still, self massage is possible but the best way is to use objects and rest your weight on them.

So imagine without proprioception you would massage yourself all day long

This is a ChatGPT answer

You would also stab and bash yourself with things all day long, man. Come on

What triggers an endorphin release when receiving touch from another person as opposed to self?

This has happened in extremely rare cases. It reveals how essential proprioception is for everyday movement. After losing his propioception Ian was completely immobilized and had to painstakingly relearn how to move by consciously tensing muscle groups. To this day every movement he makes requires complete focus and visual contact with each part he is trying to move. Basically he bootstrapped a worse form of proprioception via vision.

https://blog.oup.com/2016/06/movement-without-touch-ian-waterman/

He was interviewed on a radiolab episode which is where I heard of him: https://radiolab.org/episodes/91524-where-am-i I especially like the story of how he once saw a pretty girl and the split second of erotic mental imagery caused him to collapse on the ground 🤣

Probably because of oxytocin: the love hormone. Humans are social creatures and have reward circuits in the brain for social grooming. When we do nice things for each other, the brain releases oxytocin and it feels good. Think: a chimp picking bugs off of another chimp (also why having someone run their fingers through your hair feels so good). If you are being massaged (or groomed) by another individual, it means that you have been accepted by the group. For social animals, isolation = death.