This story was probably published in Analog, Galaxy or F&SF forty or more years ago.
Humans have left Earth, and dogs have evolved to be an intelligent, technological species. By some device, which I do not remember, a human lands on Earth. The dogs, who have always known something was missing from their lives, recognize the human as the missing, and essential, part of their emotional lives. This is 100% of what I remember about the story, except that it was very moving, and it might have been by Clifford Simiak (sp??).