New Scsientist has published lots on this. The theories abound but one that gets repeated is that the dogs, in their breeding, have been selected for puppy like behaviour - which makes them safer to be with than wolves. The timidity would have evolved over generations as wild animals just hanging around settlements. And they would have been fed scraps or scavenged. They would have been tolerated because their howling is a sign of attention and usually means danger in wild animals. Guard Dogs first, pets second, hunters third.

That's the best I can say - but I think it is the dogs danglers.