He even appears to be insecure and I'm not sure what causes some dogs to be insecure and timid. I could understand it if they'd been mistreated, but that often isn't the case. Maybe we've just been lucky, but our last dog, which appeared to be a purebred male rat terrier that we got from the animal shelter, and our current dog, a female chihuahua that we bought from a breeder, both bark when anyone comes to the door, but quit barking as soon as anyone comes in. However, instead of being timid, they've both been the exact opposite; jump on anyone trying to get them to play; love everyone and every thing.