Well, the same things cocaine or meth do. They activate the dopamine reward system and make us feel good, with a bonus of wanting to cuddle and protect a cute thing. So are kittens cocaine you can’t fight against? Yes. Expertly designed by Mother Nature to make us take care of them, it’s very hard not to adopt the kitten in trouble. In fact it is so hard to control the urge that since 16 hours ago or so when I met a kitten near the train tracks, afraid and avoiding traffic, now I have a new kitten. It took about 1/10 of a second from the moment this scared shivering alone tiny thing looked up at me when I asked “Hey little dude, whatcha doing here?” to realize… “Oh, crap, I just got myself a new cat. I am yours and you are mine Robert”…
 
But why is Robert, that fits into the palm of my hand, so cute? Because biology, that’s why. About 180 years ago the word cute developed as a slang of acute (keen, perceptive) to describe a pretty girl and was later expanded to encompass all of the cuddly, delicate youthful traits. Conrad Lorenz spent his professional life studying the phenomenon od “Cute” in living things and came to the conclusion that we are incurably grabbed by cuteness of things with small bodies and disproportionately large heads, big naive eyes, round soft heads and generally everything that looks neotonous (retaining the traits associated with youth such as tiny facial features, cuddliness, plumpness). But why, why are they so damn cute, kittens I mean. What is that strange dark purrrfect magic they have? It’s our brain forcing us into caring for our offspring so the species could go on because human babies stay helpless for far longer than most other mammalian babies do. To give birth to a baby that could relatively take care of itself by walking and doing the basics alone as most species do, a woman would have to be pregnant for 2 years and have much wider hips. These wider hips would have been a bad thing for her while she was running form predators on the savanna. So nature compromised and made women able to run and give birth to live offspring by making babies helpless for the first few years. All of the features we find cute in other living things and objects are the ones a human babies possess. Even human males have this “Awwww instinct” and are themselves a rarity in the animal kingdom with fairly maternal urges compared to other mammalian males. So this is why kittens are cute… so the human species could go on. And this is why I have a new cat, because my nucleus accumbens (the pleasure center of the brain) went bat shit crazy with primal dopamine systems when I saw it.
 
There is this whole other part where cuteness is very energetically expensive for the brain where the brain tries to compensate. punish itself, or restore emotional balance by pushing you into a form of aggression… but that’s whole other (long) story.