Seeing as the proceeds generated from T-shirts and things spawned by this video are going to shelters and other charitable causes, I have a hunch that Denver is a rescue and therefore may have had a previous owner that wasn't so nice.

Or...it could be that she's just the submissive type. My brother-in-law's dog Boomer (who I joked about hanging from the balcony when he chewed up my kilt, but I didn't mean it) does a cheesy submissive grin and has been doing it since he was 6 weeks old. He was the runt of the litter so it may have been dog-inflicted.

I like to give people the benefit of the doubt