That's an interesting question. I'm going to turn it up side down, though. Why do humans cry? We seem to be the odd species here. I don't know of another species that cries tears because of emotions. Scientists always like to figure out why one species is different.

I don't actually have an answer, but science is really about asking interesting questions, and then seeing if you can find an answer. Let's see what I do know:

I know that dogs whimper because as babies, they can get there mother's help by whimpering. Dogs are the descendants of wolves, but humans have artificially selected them for many thousand of years to keep most of their puppy behavior all of their lives. Dogs don't use their eyes as much as humans do, but their ears are better than ours. Also, puppies are often out of their mother's sight. So a sound cue like a whine makes sense. Puppies are also born with closed eyes, so tears would not be very obvious.

But is there something unique about humans that make tears useful for us? I know that human voice boxes are much lower than those of other animals. That lets us talk. (It also makes us much more likely to choke to death.) That might make the vocal part of our crying different. Why tears, though?

Researchers have found that the tears we cry because of emotions are different from the tears that we constantly make to protect our eyes. Some people think that this lets us get rid of some of the chemicals that our bodies make when we are under stress.

So why would we have evolved this useful thing (tears that make us feel better), when no other species has? I don't know, but maybe emotions have a bigger influence on our behavior. Dogs respond to things in the present, like wanting to go out or getting their tail caught in a door. We can think about how we'll feel in the future and how we felt in the past. We can get emotional about things that aren't even real, like a sad book or movie. So maybe we need tears more than a dog does.

What do you think? Your question really got me thinking, thanks.

Just about every behavior an animal has probably gives the animal some benefit, or the behavior would not have evolved.

The style of vocalizations that people have while crying is probably what most effectively communicates to other people that the person crying is in some kind of distress. The same is probably true for dogs--for some unknown reason, whimpering is the best way to say in dog language that a dog is in distress or pain. Since that's the kind of vocalization that works best for dogs as a species, that's the kind that was selected by natural selection, and gradually it became the kind of vocalization that the dog species evolved to use.

Humans benefit from shedding tears while crying because tears contain higher than normal levels of certain stress hormones that our bodies produce while were under stress. (Actually, the sweat that we produce while under stress contains more of those hormones, too.) Many scientists think that one reason why we feel better after we cry is because we have gotten rid of some of the extra stress hormones in our tears. Dogs' bodies work differently on many levels, and this is probably one of them. Chances are, since dogs don't shed tears while crying, that they have other ways of getting rid of those extra stress hormones. Dogs don't sweat either, but they do pant when they're nervous, afraid, or stressed. I suspect that the saliva they produce during times of stress contains more of those stress hormones than their regular saliva does. I don't know if anyone is researching that topic, but that's my hypothesis.

Humans whimper, too.

I suppose the better question is why dogs don't shed tears. I guess one possible reason is that when humans simply whimper, they usually don't cry; crying is caused by something else. I don't know. Interesting question.

Tears are an important protection for the eye. They can flush out dirt and keep the eye wet to help with vision. While most mammals are physically capable of producing tears, many scientists claim that humans are the only animals that cry emotional tears. But if you don't consider tears essential to crying, then many animals do cry.

Studies have found that young mammals and birds vocalize when they are separated from their mothers. Baby animals can be quite expressive in their distress at being apart from their primary caretakers. Infants of many mammalian species, including rats, cry. All young mammals make cries when separated from their mother. The cry of a bear cub separated from its mother sounds very much like a human baby's cry.

There is no scientific evidence that shows that dogs shed emotional tears. A dogs way of crying is more known as whimpering or whining. Since the dog is a social animal, it used (and still uses in the wild) the whimpering sound for communicating distress of some sort. It may be being separated from its mother or pack or if its a young individual it can even be a sign of being frightened or threatened. People who own dogs as pets will hear the whimpering sound even more often. Their pet dog might use whimpering to signal his owner that it needs something: attention, reassuring, food, going for a walk, or a pat on the head. Without proper training, it may even be a ploy to gain the upper paw,for the annoying tones of whining often cause the owner to give in to the dogs demands.