Recently PBS aired a program on "love" in the animal kingdom. It was really about courtship and mating behaviors of various animals, but it did devote a few minutes at the end to the question of whether attachments between animal mates should be called "love." The animal planet network occasionally shows programs about attachments between animals of different kinds. Some are cases of mothers adopting and nursing orphaned babies of other species, but many are animal "friendships" that provide no obvious, tangible benefits to the two parties. Surely it makes sense to apply the word "love" to those relationships.
In the seventeenth century, Rene Descartes maintained that nonhuman animals were mere biological automata with no internal life at all, no consciousness of pain or pleasure, much less thoughts or emotions. He forestalled objections that the same principle could be applied to human beings on the grounds of our capacity for complex, innovative actions. Nowadays, we know many animals have similar abilities.
Twentieth-century behaviorists would have agreed with Descartes that since we can't directly perceive feelings, only actions, we aren't justified in attributing emotions to "lower" animals. Strictly speaking, though, again on the same principle we can't assume other human organisms have emotions (or any kind of inner life) either. In practice, because we witness other people reacting to stimuli in the same ways we act when we feel certain emotions in response to similar experiences, we accept that those people have emotions like ours. We make the same assumptions about pain, pleasure, fear, anger, joy, etc. in preverbal babies (again, think of the movie INSIDE OUT), even though they can't discuss their feelings. The most parsimonious assumption, it seems to me, is that if animals react to external events in ways similar to us, they probably have similar internal experiences (especially "higher" animals that clearly have the brain structures to embody those feelings).
You've probably seen web pages that question whether your dog or cat really loves you and offer checklists of dog or cat behaviors that indicate "love." For instance, here's a slide show of actions that prove your cat loves you:9 Signs Your Cat Loves You
Personally, I don't believe our dog and cats snuggle up to us only because we feed them. They show affection to everybody in the house, not just the habitual food-dispenser (me). I vote for the "love" label.
Margaret L. CarterCarter's Crypt