When the loved one gently puts his/her head on your shoulder, there is smile on your face with the thought, I am always here for you. This expression is like a smile, and then there is caring thought going on. When you do it, it's not a proper smile, but a half smile, but gentle. Like when a kid comes to you and puts his head on your shoulder, you'll have this expression. It's a smile not to show to them, or anyone, it's just you do it as if you mind is programed to do.
I don't know if there is one specific word for this exact situation. However, the first thing that popped into my mind was:
- a warm smile
Interestingly enough, in its definition for warm, Macmillan says:
warm (adj.) kind and friendly in a way that makes other people feel comfortable : a warm smile
Wordnik also says:
warm (adj.) intimate; close: as, warm friends
warm (adj.) friendly and loving:
They're a very warm family.
He has a lovely warm smile.
Again, I wouldn't say that warm is the definitive correct answer, but it is one possible expression you could use.