I have a Guinea pig who is about 1 year old now

I've had him since he was about 5 months. I was wondering why he started to nibble/lick my skin when I rub him? It's like my dog, when I rub him he usually scratches and puts a smile on his face. I'm not hurting either of them but just curious as to why this happens. If I keep rubbing will he nibble harder? I really can't explain if he's licking my skin or nibbling on it as it doesn't hurt but I can feel he's trying to pinch it. Do Guinea pigs have rough tongues like cats?