The animals that became domestic dogs speciated from the gray wolf about 15 millenia ago. Since then, our dogs have come to know us well.

Dr. Deborah Custance and Jennifer Mayer, of the Department of Psychology at the University of London, sought to discover how well in an experiment chronicled in the May 30th issue of the journal Animal Cognition. What they discovered is that dogs are overwhelmingly more likely to approach people who exhibit visual and audial signs of distress, such as crying, than people who are talking, humming, or being quiet. And when dogs approach weeping humans, they will tend to assume submissive, friendly, comforting postures.

From MSNBC.com:

"The humming was designed to be a relatively novel behavior, which might be likely to pique the dogs' curiosity," study researcher and psychologist Deborah Custance said in a statement. "The fact that the dogs differentiated between crying and humming indicates that their response to crying was not purely driven by curiosity. Rather, the crying carried greater emotional meaning for the dogs and provoked a stronger overall response than either humming or talking."

… The experiment took place in the [dog] owners' living rooms. Mayer would arrive and ignore the dog so that it would have little interest in her. Then she and the owner would take turns talking, fake-crying and humming.

Of the 18 dogs in the study, 15 approached their owner or Mayer during crying fits, while only six approached during humming. That suggests that it's emotional content, not curiosity, that brings the dogs running. Likewise, the dogs always approached the crying person, never the quiet person, as one might expect if the dog was seeking (rather than trying to provide) comfort.

"The dogs approached whoever was crying regardless of their identity. Thus they were responding to the person's emotion, not their own needs, which is suggestive of empathic-like comfort-offering behavior," Mayer said in a statement.

Of the 15 dogs that approached a crying owner or stranger, 13 did so with submissive body language, such as tucked tails and bowed heads, another behavior consistent with empathy (the other two were alert or playful).

The study's authors are quick to point out that just because dogs appear to feel empathy with humans, it does not follow that they actually do.