system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does it feel so much better when someone else uses one of those scalp massager things on your head compared to using one on yourself?

I guess the same question could go for a lot of things, but I'm sticking with the head massager for this example.
gpt
It's expectation vs. reality mismatch. There have been studies done where humans operate a robotic arm and use it to tickle themselves. If the arm moves with absolute precision, the subject does not feel "tickled." If the robotic arm's movements are delayed by a couple of miliseconds, it tickles. The brain automatically models what it expects to happen. But you can't precisely map another person's actions - even if you agree on the pattern and timing, you'll likely be a few miliseconds out of sync. It's like "situational irony" in the body.