OK you are talking more about the wet teariness of it. Here’s my wag, we are made to be social creatures, made to help each other. In times of extreme emotions, and emotions are a critical part of us, we cry as a external recognizable signal for assistance of others, though we may no longer wail, and even try to hide our crying, but wet tears are something hard to hide, and it is to our benefit to have help at those times.
Why it’s crying as opposed to a LED indicator that we need help, I don’t know.
I don’t have cites but it’s clearly an effective “I need help” signal. People cry when they feel they can’t handle something alone. That’s why women cry more often than men and why it’s sometimes seen as effeminate for men to cry - it’s a sign that you can’t handle your situation alone and you need someone to help you. It’s generally automatic and subconscious which is how you realize the person really feels like they need help - think about how surprising it can be when you realize a friend is crying over something, and how you instantly change the way you act towards them in that situation when you realize how upset or worried they are. (That’s also the reason we find it so unnerving and horrible when someone, like a con artist or sociopath, fakes tears to deceive us). I don’t know why the “I need help” signal specifically involves tears or reddening of the eyes, but I would guess the reason that very few other animals cry in that way is because animals don’t tend to study each other’s faces as closely and frequently as humans do.
In terms of grieving: chimpanzees supposedly grieve over their young when they die, there’s pictures here. I think I’ve read that certain other apes do as well.
Yes, that’s exactly how young I meant. Newborns AFAIK don’t actually shed tears. And somehow seeing a baby cry real tears is more pity-inducing than just the wailing.