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.