Why does John immediately like Mr. Pignati in The Pigman?

When John and Lorraine first call Mr. Pignati, he seems excited at the simple notion that Pignati is gullible and could easily be taken for a ride. So he does not necessarily like Mr. Pignati himself, but only the possibilities he presents.
When they visit Mr. Pignati to collect the $10 he had promised to donate to their made-up charity, John loves the big goofy smile Mr. Pignati is wearing. He relates the "crazy" smile to a drunken Santa Claus. He and Lorraine both think he is "too nice to be for real" and although later he considers that Mr. Pignati might be a psychotic killer, that smile on his face is what immediately makes John like him.