Why do dogs tilt their heads? New study offers clues
Or maybe they to it because we think it is cute.  The dog's evolution is totally tied to their interactions with humans.  The head tilt is a form of communication, them telling us that they are doing a particular behavior.  Those that head tilt would better communicate with people, resulting in better treatment, more breading opportunities and an evolutionary pressure towards more head tilting.These are border collies, a working dog designed to interact with a human handler.  Take a look at something like a cane corso or great pyrenees.  They don't head tilt nearly as much because such close communication is not what we have, through breeding, designed them to do.