Sheldon is a douchebag because he is brutally honest. He doesn't have that part of personality that softens one's opinions for the benefit of other people's feelings.
Leonard doesn't want to discuss science with Sheldon because Sheldon thinks lowly of Leonard's scientific merit, and doesn't hesitate to express that sentiment (see above).
Right, I get that. But it is not honest to ask someone "where do you stand on the anthropic principle?" if you assume that they do not know what it is. Asking in that way is a common trickery to get someone to look foolish by attempting to answer a question they have no idea about. Classic douchebaggery.
Leonard doesn't want to discuss science with Sheldon because Sheldon thinks lowly of Leonard's scientific merit, and doesn't hesitate to express that sentiment (see above).