I imagine that dogs throughout history tended to have a role which were more important than just keeping humans company? Why and when did that change?
Can you clarify the question? Are you asking when house or companion dogs became popular? Working dogs are still utilized in many fields. While "companion" dogs are found in a number of civilizations in history even they often served a purpose, whether it be to raise an alarm, deter mice etc. I can provide more information and point you to some resources if you can narrow the question.