I just read somewhere that this is a sign of affection. I can't remember where I read it, or whether it is from a reputable source. But out first dog, Chinook (nn), did this when we were petting him.
Vin does this with Zeke....accept he says it's a spider....and it's just their interaction and I always love it.We have a game we play where I make my hands look like the evil claw. I even say, "Oh no, it's the claaaaawwwww". She immediately growls and starts kinking her front paw at it. I act demented and we both go into character. It's so fun because it's our thing.
Gracie puts her paw on my arm when she wants me to pet her...I'll pat, then stop...and the paw comes back again, over and over....