system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do so many species enjoy being rubbed or scratched?
gpt
This has to do with sociality. In humans and other primates grooming is an important part of social bonding. In dogs and cats it may be more to do with having co-evolved with humans. If the dog likes to be petted then it will be friendlier and will be selectively bred. Cats have a natural urge to rub their heads on their friends. We see this in pretty much all cats. Some of that behavior comes from cats having scent glands in front of their ears, so they are rubbing their scent on their friends. So it's basically a way for a social animals to strengthen and reinforce bonds among their group-mates.