[Help] why does dog lick our faces so vigorously before bed?

Every night, our 1.5 year old maltese decides he absolutely has to lick our faces for about ten minutes after we get into bed, or else it's like the world will end. We call it goodnight kisses, but he's so intense and earnest and deliberate about it, I wonder if there is some biological or instinctive reason why he does it. It's so intense! Any ideas? via /r/dogs http://ift.tt/2rzvySA