> Oddly, tameness has also long been associated with traits such as a shorter face, a smaller head, floppy ears and white patches on fur—a pattern that Charles Darwin noted in the 1800s.
Hmm, so evolutionary pressure of existing around humans makes animals cuter.
I believe the main biological lever is retaining juvenile features as adults, physically as well as mentally (like with dogs). What we see as cute is an honest signal that they are more child-like: less aggressive, more trusting and pro-social.
I also think that this is the central cause of a wide variety of domestic/cute adaptations. There are too many separate features to believe that raccoons and dogs and cats and a dozen other species all select for these same elements independently.
I no longer have the book on hand, but read a few months ago that this correlation between juvenile traits and domestication was one of the main theses of Barrett's "Supernormal Stimuli" in Chapter 4. She cited a few studies of fox domestication [1], [2] and other works to support these theories.
My guess: possibly co-evolution. The article subsequently describes the genetics behind things becoming cute - which would have been completely benign to our ancestors (the core of your question). However, those of our ancestors who completed domestication of these animals (by random chance) would have enjoyed more protection from predators, rodents, etc. Those of our ancestors who attempted to domesticate things without the mutations might have had bad companions at best, and would have been predated at worst. This would have provided evolutionary pressure to adopt animals that were showing early signs of domestication. What we call "cute" is merely "likely to cooperate with us."
Since humans associate cuteness with large eyes and small body size, nocturnal / twilight animals, like raccoons, sugar gliders, cats, squirrels, etc have a larger chance to be domesticated as pets.
Daytime, larger animals (e.g. sheep, goats, or even rabbits) have a larger chance to be domesticated as food.
Experiment in taming foxes that relied on a single principle of selecting animals that react to humans with least fear and aggression resulted also in those morphological changes. I think it's more about selecting animals that retain youthful curiosity and other traits into their adult life. Youthful morphological features just tag along.
Perhaps a combination of adaptableness, small size, prodigious reproduction, and cuteness saved some species from being wiped out whereas other species didn't fare so well once humans arrived and transformed their territory. Adapt to urban encroachment or face extinction.
Hmm, so evolutionary pressure of existing around humans makes animals cuter.
I wonder why we find these features endearing?