system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do dogs react a certain way when you scratch certain spots?
gpt
It's stimulation of different sensory neurons. 

It's like when a woman holds your hand vs. your penis. 

Yes, holding your hand is nice. You like the feel of it and you smile. But when she's holding your penis—even if the same grip and pressure—you get a happier, more pleasant feeling that makes you pant loudly and your leg wiggle.