Why did so many people invest in the stock market in the 1920s?

The 1920s was a time of enthusiasm following World War I. They had the sense that anything was possible with new inventions coming out, like the radio and airplane. Although the stock market has always posed a risk, the optimism of the people in the 1920s was too great; they ignored it. They pulled money from everywhere, investing their entire life savings.
Because the governments of the world printed a lot of money during WW1 to pay for the war, the inflation didn't catch up until the 30's, which made the "roaring twenties" possible.