A simple rabbit expression of happiness is the “shudder.” The rabbit will lift its front legs up a bit, and shiver the skin of its chest and back by twisting rapidly. Sometimes its head and ears will twist a little too. A rabbit shudders to tell you that it is happy, hopefully with you and in general. You can think of it as a glad pat on the back for a job well done.
There does seem to be one common exception to the shudder as an endorsement of your charms. Rabbits that don’t like being held or confined may give a shudder when released, presumably because they’re relieved to be free of their confinement. An easy way to recognize this version is that it’s often accompanied by parting foot-flicks as the rabbit hops off.
Although our skin isn’t loose enough to do a proper shudder, you can do a passable version if you are wearing a dress or long bathrobe. A quick twist of your body back and forth once or twice will make a clear statement, and is certainly a polite response to being shuddered at. Or you might prefer to respond with an even happier “head-flick” as described in the next section.