Probably just mammals; I’ve owned fish and iguanas and they’ve never wanted to be pet. In mammals, physical, non-violent contact releases oxytocin in large amounts. It’s basically a drug to let us know we’re all right. I bet if you didn’t have tv, Reddit, a gf or other hobbies, getting a little affection would mean a lot. As it stands, our animal slaves get treated like animal slaves rather than non-human persons, so we are mystified by them getting excited by simple things (seeing another dog, not being left to starve, etc).