My youngest one Almond licks me all the time, even if I had just washed my hands and have no food on them. He licks everyone, my hubby, my mom, dunno about my dad tho... lol. He's a licker, and seems to like to want to groom me. He'll even hold onto my hubby's hand when he pulls it slowly away and will still be licking. It's like Almond is saying "Wait, I'm not finished!" lol. So cute. xD
My other guy, Peanut, is a dumbo, and he doesn't lick unless I have something on my finger or hand for him to lick. lol.
I think it can be a collection of things. Like in dogs, they lick for more than one reason. Dogs lick when they're happy and when they're stressed. If they're stressed, they lick in order to keep themselves calm. I think rats probably do the same thing, so it can be either or- or a combo- like a "I'm a little stressed, but I love you too, so here's some lovin'." lol!