There are a lot of things science can't explain. All I can say is what I've learned from my personal experiences and education over the years.
Perhaps I'm misunderstanding what you're trying to say here, but it came across as pretty hurtful to me. It's as if you're saying I have the emotional magnitude and differentiation of an infant. It's not like I see someone crying and can't help but burst into tears. I've felt emotions that aren't my own while I'm sitting at home reading, only to find out that my neighbor was distraught after learning that his brother had been killed. Not sure how else that can be explained?
If you want to take a scientific stance and think that anything not proven by science isn't real, that's fine. However, I don't agree with holding that as the ultimate and solely correct worldview without acknowledging there could be other explanations.
Perhaps it's a different phenomenon that we are thinking of. I have known people who do have what you just described, "see someone crying and can't help but burst into tears" as adults, and I would not say that they have the mental fortitude of an infant. They just have a brain which doesn't work entirely the same as other people's. Developments happen to your brain as you progress through life. For some they happen later and for some, some developments don't happen at all. We can understand these things easier by comparing them to younger humans and animals.
I'm sorry for how I answered though. It was an insensitive response.