Why are humans attracted to baby animals?
Let me describe what a baby is like to a man. It smells funny. It doesn't talk. It's helpless. It's FREAKING ANNOYING. It consumes your resources. It consumes your wife's time. It hijacks her breasts. It traumatizes her female parts, putting a hiatus on your happy-fun-times with her. It competes with you for her attention. It literally shits itself. It wakes you up at night - every night - for months. For the first few weeks it doesn't smile or reward your behavior in any way.
It is, quite frankly, ten times worse than the worst roommate you can even imagine and no, it doesn't pay its share of the damned rent. Or anything else. All those bonding hormones from breastfeeding? Yeah we don't get those. The months spent nourishing this unborn child, feeling its every kick, questioning every meal or odd physical sensation? Even the most devoted father simply doesn't get that. We're just not physically built for it. We're fathers, not moms.
Nature stepped in here, because you'd kick the snot out of a roommate like that and it didn't want us making ourselves extinct. Enter cuteness.
I could go on and on pointing out what things make something cute. A big head, for example. Big eyes. Wonky proportions. Etcetera. The thing is, I don't have to - you're human, so that stuff's already hardwired into you to one degree or another. The interesting thing is that those traits all fade with time. Legs with those proportions are necessary to maximize space in the womb, but they're impractical for running. That giant head was necessary for early brain development, but impractical for... well, damned near everything.
There are strong arguments that neoteny has been selected for as a result of younger looking juveniles receiving less severe punishments, the need to be more selective about mates in an environment when some females were post-menopausal, etcetera, but that's going a bit deeper into the anthropology than we need to. The point is that we don't tend to kill cute things, and as any parent will tell you children - especially babies - definitely needed something to stop us from throwing them out the window after the 20th straight day of not getting more than an hour or two of unbroken sleep a night.
Cuteness is that thing. It's hardwired to trigger a "take care of me - I'm cute!" response. The same mechanism is seen in other animals to some extent. Fun fact - predators looking at cute animals of other species see "Come eat me - I'm helpless and delicious!" So you see cuteness is a universal signal.