It's plainly axiomatic. Humans do this because it is valuable to treat other humans as if their pain, injury, or death is important. The other humans extend reciprocity. We will, for instance, chase your murderer down, because if the situation is reversed, we're pretty sure you'll chase our murderer down. We get a deterrent effect for practically nothing... that's a bargain.

Mind do not extend reciprocity. They will not chase your murderer down, or avoid injury to you.

If you're searching for more than that, you've turned it into a religion. This is a practical matter.

It's unlikely that you have developmental issues that makes this impossible for you to understand intuitively. I suspect that you were miseducated.