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.
Oh my gosh oh my gosh I have so many questions what is LadyBug really like? She's just so amazing I love her can I meet her I want to marry her omg I'm freaking out by the way I'm Adrien I can't believe I met Ladybug's Kwami!!!
wait you're that boy Marinette lik-HOLY CRAP HOW DO YOU KNOW ABOUT KWAMIS
uh so what exactly are you?
my name is Plagg and I'm Chat Noir's Kwami
oookayyy do you want a cook-
it's Adrien 100% Adrien he is Chat Noir totally him I have proof yep lady Adrien Agreste is Chat Noir deal with it also screw cookies where's the cheese