It's his way of amusing himself.

He amuses himself at our expense," the girl continued.
"Well, I hope that makes up, my dear, for the rate at which we're doing so at his!" "His amusement," said Nanda, "is to see us believe what he says." Mr.Longdon thought a moment.

"Really, my child, you're most acute." "Oh I haven't watched life for nothing! Mitchy doesn't care," she repeated.
Her companion seemed divided between a desire to draw and a certain fear to encourage her.

"Doesn't care for what ?" She considered an instant, all coherently, and it might have added to Mr.Longdon's impression of her depth.

"Well, for himself.