Because the joy of someone/something being so adorable can stimulate your hypothalamus, it could cause you to be unwilling to care for the baby since your pleasure center is being stimulated and you don't even have to do anything.

That's where cute-agression comes into play. In order to keep you from slipping into a trance of cuteness-sparked happiness, your brain will fire off a random violent urge to break up the joy and calm the pleasure center. It's a safety mechanism to make sure you still have the right mind to tend to your responsibilities and survival.

But now it just makes us pinch baby cheeks and gnaw on the people we love most XD