Some people say that it's "dominance" when a dog puts a paw on you while you're petting him, but I think that's hooey.
There could be a lot of reasons. If he knows a couple of tricks that involve giving paw, he may think you like it, so it could be a kind of "thank you" for the petting.
Also, a dog may show gratitude or submission by rolling over during a petting session. If he's sitting, he may put a paw up and tilt his body, almost like he's going to flop down and roll over, even if he never actually rolls.
There could be a lot of reasons. If he knows a couple of tricks that involve giving paw, he may think you like it, so it could be a kind of "thank you" for the petting.
Also, a dog may show gratitude or submission by rolling over during a petting session. If he's sitting, he may put a paw up and tilt his body, almost like he's going to flop down and roll over, even if he never actually rolls.