I have a professor that brought up a point once that I think was interesting. With animals, intelligence and affection are linked. More intelligent species will show more affection because it's a form of mental stimulation beyond just survival. I think it makes sense.
Most fish I've kept hardly even noticed there were humans around at all; bettas not only acknowledge your existence, but they interact as well. Hamsters tend to not care much if you're holding them or not, but rats are very sweet and attach to their owners. The same can be said about finches versus parrots, or dogs versus hermit crabs, etc.
He also brought up herd animals versus pack animals where packs have visible social structures, etc.