My Yorkie is very friendly, boy I hope he'll stay that way. He's only growled at his beloved flossie (that's almost gone anyway) while playing with it.. My mom knows somebody who bought 2 sisters from the same litter and one of them hates everybody and everything! Even her own sister.... And they are only about 6 months or so. My mom brought her boy for a playdate and that fiesty little one wouldn't stop growling and lounging and biting. They had to separate them
. They tried to socialize her and all but she's been like that from the beginning. She even bit her owner so hard the lady was bleeding... It must be just aggressive personality.