The Underground Man goes back to his technique of filling in our side (the reader's side) of this "conversation." He supposes that we might be scoffing at him, asking if he intends to prove there is enjoyment in a toothache, too.
Well, now that you mention it…Yes, he does intend as much.
There is enjoyment in moaning over a toothache, he says. If moaning weren't enjoyable, than the man with the toothache wouldn't moan.
What the moans do is express the purposelessness of the pain. As he discussed at the end of Chapter Three, pain is all the worse when you have no one to blame for it, "no enemy to punish."
If we don't believe him, he recommends that we go and listen to a man suffering from a toothache. At about the third day, he's not moaning out of pain, he's "only amusing himself" while making everyone around him miserable.
The Underground Man figures that, at this point, we're laughing at him. He resigns himself to this, admitting that his words are in poor taste, jerky, and lacking self-confidence. But that, he says, is because he doesn't respect himself.
He ends with the question, "Can a man of perception respect himself at all?"