[I_AM_A_SMURF]

We're programmed to take care of (human) babies. That's pretty fundamental to our species survival.

Those features activates the same areas of our brain that babies' faces activate.

Feeling that something is "cute" is the evolutionary way that our brain is using to make us care of our kids.

[actionfromafar]

Put slightly differently, they look cute because we are all mammals and they are cute.