In the dog world, usually a dog will paw at another dog (or give paw) when it wants to play. How the other dog perceives the dog giving paw determines the interaction between them. If the dog assumes it as a play move then the dogs begin a play session. If the dog interprets it as threatening it will either back away or a fight will occur.

Assuming your dog is well trained to humans touching it, having your dog give you his paw is a good thing. It indicates your dog enjoys your touch and desires more of it. So basically your dog is saying "I like that feeling when you rub or touch my paw, please do it again".

I do not believe it is a dominance issue although I have heard others say it is. My Dixie will have you rub her paws until they fall off if you let her. When I rub them for her you can see the look of pleasure it gives her when you look into her eyes. When I tell her "NO PAW" she puts it down and is content to have me touch or pet her some where else. But her paws are her favorite things to have touched. I believe letting a stranger rub her paws shows how comfortable she is around humans and how much she loves them.