Our skill to empathize and understand each other is not innate. It is learned in the early years of our lives. We gather tools and skills like emotional understanding, intention, and empathy over time to develop what is called a Theory of Mind. According to developmental psychologists such as Wellman (1990), Theory of Mind is the skill to infer and attribute intention, behaviors, thoughts, and emotion to individuals other than one’s self. Theory of Mind is a crucial piece to the puzzle of understanding how we as a species conceptualize "self" and "other" within the brain.
See the difference between Theory of Mind and no Theory of Mind:
The video above shows the "three mountain task", a task given to a child to test their sense of egocentrism, or their grasp of a Theory of Mind. In the video, the first child does not understand that the adult sees something different than what they do, and so reports that the adult can see everything the child can, which is not possible. The second child, however, has grasp of a Theory of Mind, and correctly identifies the objects that the experimenter can see.
The difference is stark. Once a child attains a Theory of Mind, he/she/zi can conceptualize someone's perspective and answer questions like "what is that person feeling?" or "how would I feel in that situation?" Before gathering tools such as empathy or intention, however, children as old as six still cannot understand the perspective of others'. But even if we know when we begin to empathize, how does it actually work? What happens inside our brains that allow us to reach inside someone's mind and pluck out their emotions?