I think a lot of it might be their personality. We've had three yorkies over the past 20 years and they've had three totally different personalities. Our first, Macho, never met a stranger. Canine, feline, human, everyone was his friend. He'd beg treats, give kisses, play with anyone. Fuzzy, our second, didn't really care. Her attitude with other people and animals was ok, you're here. If you want to pet me that's cool, if you don't, whatever. Our little girl now, Sophia, is uber clingy. She won't play with other dogs. If people come to the house she either barks her head off from behind my legs or she asks to be picked up and then refuses to look at them. Guess she thinks if she doesn't see them they don't exist
. When we're out at the park, Petsmart, etc. she'll approach other people and sniff, won't approach other dogs though. And any cat she sees other than her "sister" is lunch.