Personal Science
There are a lot of other things we figure out during that time of life, as well. Basically, we are learning the rules of this new game we've been thrust into, called Life. Once certain lessons are learned, they can be internalized and you can stop testing them. Like gravity. It's pretty reliable for babies on earth. We take it for granted. In fact, if it isn’t learned and internalized, chances are that juvenile human will die.
...that empathy is a limited and local resource. Expanding one’s desire to empathize with an increasing number of people spreads it thin like too little butter on too much bread.