Scientists studying the physiology of joy have made a startling discovery: rats are actually ticklish. And when you tickle them, they actually laugh. Of course, their little rodent guffaws are in the ultrasonic range, so it actually sounds like R2-D2 on a meth bender.
Two things come to mind here. First, given the, uh, vigorous nature of the 'tickling', it's possible the rat is actually screaming. Or delivering some variation of the line, "Dear God! The hand! The hand has returned!" Or possibly even "Stop tickling me, you perv. It's a form of abuse."
Second, there is a man out there whose entire job is tickling rats. All he does, from the time he gets to work until the time he leaves, is touch rodents in an amusing way. He may even think about tickling rats while at home with his wife and children. And if that doesn't put human existence into some sort of perspective, then I don't know what will.