Why does Charlie begin to spend a great deal of time alone in Flowers for Algernon?

In the book Flowers for Algernon Charlie finds himself becoming less and less accepted by people the smarter he becomes.  His superior intelligence even serves to alienate him from the doctors who had performed the experiments on him and Algernon, the mouse.  He finds that people are uncomfortable around someone smart and he feels alienated.  He goes more into books and studying since his presence now intimidates people.

When his intelligence begins to deteriorate later, Charlie locks himself away from people because he feels frustrated, ashamed, and upset.  He is also experiencing increased episodes of anger.