Baring teeth on one side -weird or worrisome?

[Scarlettsstars]

My golden retriever will raise his lip on one side of his muzzle when I stroke his cheek. Stroking the other side doesn't provoke a similar thing - it's like a lop sided sneer. I've been figuring its an autonomic nerve twitch - like when you manage to get their 'scratch g-spot' and the back leg starts waving involuntarily. He shows no signs of being upset about it, nor any signs of dominance/aggression. Should I be worried?

He's 4 by the way and has always done this, not to drip feed.

My mums dog has a nerve on both cheeks that will make him pull up that sides lip. It may be a nerve have a look at all his other body language signs though. x

I have had a lab and a cocker that can smile, looks very worrying for people who didn't know them.

Thanks Claire and Cath, I'm not worried about him but sometimes I wonder! It's impossible to imagine him being aggressive - he's a soft lump- but then suppose everyone says that so I was just looking to see if others have had similar with their dogs.