Not everyday is a good day. Adrien knows that. He also knows that his purring is useful to calm people (especially children during an akuma attack. Kids like the sound of it and that way he can do his job quickly).
So when Marinette has a bad day
he tries to do the same she does when he feels down: be there for her. And he use his purr to calm her a little. This purr is lower and longer than the ones he uses for kids, since he is not in a hurry and wants to express better what he feels.
Also forehead touches are a Marichat thingy, fite me.