What is the theme of the story "Flowers for Algernon"?

There are a number of major themes present in Daniel Keyes's novel Flowers for Algernon. One of the most thought-provoking is the nature of happiness. Before Charlie undergoes his experiment, a strong argument can be made that he is happy. He has a job in a bakery that he enjoys. He has what he sees as pleasant interactions with his coworkers and friends. He values the education he receives from his teacher, Alice.
Then he becomes smart. He undergoes an operation that causes a huge increase in his IQ, and he realizes the reality of his situation—that he was looked down upon by the people around him. As his intelligence grows, he alienates those he cares about. In both situations, Charlie is exceptional. At the beginning of the novel, he is exceptional due to his lack of understanding. He then becomes exceptional because he gains so much understanding. He is not, however, happier with this increased intellectual ability.
It would seem that Charlie is happier at the beginning of the text—happy to be in Alice's class, happy to laugh along with his coworkers as they make fun of him. Once he gains understanding, he drinks, gets in fights, and becomes incredibly depressed. When Charlie begins to revert to his former state of intellect, it may seem at first to be a positive thing. But because he remembers his genius, he feels shame at the idea of being pitied. He cannot revert to his former state of joy.
This text almost seems to be a biblical allegory for the story of the Tree of Knowledge from Genesis. Much like when Adam and Eve eat from the forbidden Tree, Charlie is cast out of Eden—he loses his innocence. Even though he tries, he can't regain that simple happiness he once had. This text asks us to question, using the extreme example of Charlie, the extent to which knowledge can give us joy. For Charlie, it can't; it only brings him misery.