Rabbits grind their teeth together in two different ways. Both ways mean something very different. When rabbits are comfortable, they will grind their teeth quickly, in short bursts.
But, if the rabbit is in pain, they will grind their teeth very slowly. This is one of the only ways that we can tell if our little furry friend is in pain. Being a prey animal, they are very good at hiding their pain because they don't want to be picked out as a weakling or an easy meal.
Not all bunnies will grind their teeth when they are comfortable or relaxed. Some will lick you, or do nothing at all. It all depends on the rabbit.