Keyes, Daniel. Algernon, Charlie, and I: A Writer’s Journey. Boca Raton, Florida: Challcrest Press Books, 2000. (Contains the short-story version of “Flowers for Algernon,” originally published in the April 4, 1959, issue of The Magazine of Fantasy and Science Fiction.)
Small, Robert, Jr. “Flowers for Algernon by Daniel Keyes.” In Censored Books: Critical Viewpoints, edited by Nicholas J. Karolides, Lee Burress, and John M. Kean, 249–255. Metuchen, New Jersey: Scarecrow, 1993.
Charlie's "friends" laughed at him because he was cognitively impaired, and in the beginning, he wasn't really sure why so he just laughed along with them.