i do not know for sure, but i think it is analog with the reason why you are smiling…
it is the expression of soul. maybe they are the media to expres our feeling.

I have always regarded crying and smilying as expressions that evolved for social interactions. They might be a form of metacomunication(tell everyone that the predator is here and took a bite out of me) or a sort of a child to call his mother if he is injured. Also, tears are a form of excretion…

I always associated crying with an emotion. When you are upset your brain is using electrons in one of the lobes. You cry because your brain is attempting to express it and it sets of the tear ducts. It is a bit more complex but I believe that is the basic.

Lol… don’t use the term emotion, it has no scientific definition. Or at least not a physiological one. It is a phychiatric definition and not important for us. Second of all, how exactly does the brain use electrons? Look up neuron function and the importance of Na+ and K+ ions. No free electrons involved and they also aren’t being used, just moved around. Third, the real question is why your brain might "want to express itself" using tears?

We cry when we are injured because we have pain receptors that tell us when tissue is damaged.