Funny! Our neighbors dog used to look like she was growling because her lips would go back and her teeth would come out. But her tail would wag the whole time! It was quite funny once you found out that she wasn't growling or going to bite you. Instead she just had this quirk that made her look mean when she really was happy and just wanted you to pet her.
She died a while ago but I still remember the funny "grin" she had! My kids were afraid of her back then. But, they were still quite young so they were sure she was mean. ;-)
I love how dogs seem to use their charm to get us to do their bidding! But who can resist that?! Especially when they are so excited to see us or love on us!!!