Although cuteness can be a matter of personal preference, people tend to find young mammals cute and adorable, and this is likely part of an evolutionary development. Cuteness serves a purpose in species whose young cannot live on their own right away. People are drawn to babies, and it’s their body proportions (large head, large eyes, and small mouth, compared with the rest of the body) that may be an evolutionary trigger or a reminder that nurturing our young is an important adaptation for our long-term survival as a species. Babies are helpless, so our affection for them and our instinct to protect them, regardless of whether we are related to them, makes good evolutionary sense.
The young of other animals that also require some level of parental care tend to have similar facial and body proportions, and these qualities make it more likely that their own parents will provide sustenance and other resources long enough to ensure their survival. Some scientists argue that the same adoration we feel for our own young can carry over to the young of other animals who require parental care and have body proportions similar to our own young. These qualities appear in mammals mostly (especially in pet dogs and cats and in mammals at zoos and aquaria and on nature programs), but people can be drawn to baby lizards, frogs, and birds as well. Austrian zoologist Konrad Lorenz coined the term Kindchenschema ("baby schema") to describe this phenomenon.
In animals that are independent from birth or hatching, cuteness is not a priority. The head and body proportions of the young of these animals may be nearly identical to their tougher-looking adult forms, and this may be why human affection for young insects, snakes, and even some types of birds is not as great.