Check out this site to learn the background of each of his works, including the historical context in which each was written and his personal preoccupations at that particular time:
According to the National Steinbeck Center, John Steinbeck attended Stanford University as an English major, already having declared a love and talent for writing. He did not complete his program, however, and instead left for New York to pursue his writing career. It's hard to determine exactly his reasons for writing; however, his writing does show that he wrote based on experience. He incorporated jobs he had worked (such as working on a farm in "Of Mice and Men"), as well as places he had visited and/or lived in. His writing also expresses the sentiments of the country during the Great Depression, so it is very probable that he used his writing as a vehicle by which he could express the American experience during this time, as well as the human condition in general.