My 5 month old GSD puppy does this to me whenever I rub her spot on her tummy. He is just showing you affection and unless it bothers you, it's okay. A word of warning though, if he becomes too protective of you such as not letting your fiance show you affection then he see's you as his mate and the behavior should be stopped. But unless this happens (and it probably won't) then I wouldn't worry about it.
