[alecperkins]

It's a running joke that Sheldon and others look down on Leonard as a scientist. The claim is often that his work is very "derivative". Even Leonard's own mother criticized the originality of his science fair experiments as a child. Also, the dynamic between Sheldon and Leonard can be seen as theoretical physicist versus experimental physicist. (Not to mention extending the layers of superiority when Howard, the non-PhD mechanical engineer, is included.)

As for Sheldon generally being a douchebag, the article does a good job of explaining his (partially deserved) sense of superiority.