Am I my dog's "mate"?

So I own a 5yo. neutered German Shepherd, he is protective, obedient and he knows I'm dominant but the calmer pack member. My sister is like a play mate to him, she is constantly at his level and dog-like wrestling him. Recently he is constantly following me for food, I shared a little too much with him and to remind him out of my space I've been demanding him downstairs, out of my way. But for the last week when he is tired he hops onto my bed then snuggles really close and nibbles my hair and back while groaning...He has been doing this since he was a pup but instead of seconds it lasts for minutes and he is doing it constantly. He did do it to my sister but she would have to ask by putting her head between his paws...Last night I was minding my own buisness while he was grooming me thoroughly softly for ages, then I turned and rubbed his ear and he moaned really loud. So...What is this?! Lol.