Common reasons why your dog might be licking your head are that it has learned that the behavior is rewarded, it wants extra attention, it is trying to show affection, or that it is doing it due to excitement.
Why do dogs lick me when I lay down?
Affection. The reason why it does it could be that it is being affectionate. This would be more likely if it does it in response to you being affectionate towards it such as when you are petting it. When it’s licking you it’s letting you know that it trusts you and that you are a part of its tribe.