all living organisms benefit from affectionate treatment
mammals are the only ones that I know that seem to feel emotions and display them, usually cause it seems their brains are a bit more complex in the emotions department. this was probably evolutionary, since they had to take care of their live born..
I mean, there are reptiles and birds that stick around with their eggs till they hatch, but once that's done, they might eat their young...
also mammals tend to have social organization, either a wolf pack, or a lone polar bear mother and her cubs.
also mammals play with eachother, often in an affectionate way.
I guess what I'm saying is: because mammals show to have a better developed sense of self, as individual and as individual being as part of a community, they also have emotions and show affection to others.
Before you can offer love, you have to know you exist as an entity.
without that, it's like asking: can John give you chocolate from a box that John has no notion of it's existence?
non-mammals seem to be much more instinctual, which is what happens when you have limited/reduced or no sense of self
EDIT: John won't even understand what chocolate is since he cannot understand what box containing chocolate is.