What is the American Dream and why is it so important to people?

This phrase, first expressed by James Truslow Adams in 1931, is the idea that many of the immigrants who came through Ellis Island clutched emotionally.  Having left their own countries because of poverty, war, or civil or religious oppression, they felt that if they worked hard enough in America they could achieve a quality of life superior to the ones that they had left behind on the other side of the Atlantic Ocean.  This idea is rooted in the second sentence of the Declaration of Independence, which states that "all men are created equal," and that they are endowed with "certain inalienable rights." 

Fathers who were vegetable peddlers have dreamt of their sons being doctors, paperboys have dreamt of owning silver mines, a bobbin boy in a cotton mill has dreamt of owning ships.  People in the tenements of New York dreamt of owning homes of their own. Before them, indentured servants,  slaves dreamt of their freedom.

The American Dream was certainly realized by men such as J.D. Rockefeller who owned Standard Oil Company, and Andrew Carnegie, the steel magnate who literally rose from "rags to riches."  Another such man who reached the American Dream was Cornelius 'Commodore' Vanderbilt, whose great-great-great-grandfather came to America as an indentured servant.  Vanderbilt, who quit school at age 11, worked on his father's ferry and went on to buy steamboats and railroad.  He is the third wealthiest man in American history after Rockefeller and Carnegie.

In "Mice and Men," by John Steinbeck, the American Dream is what sustains George and Lennie in the depressed era in which they live.  All their dream consists of is a ranch of their own and some rabbits for Lennie to pet.  But, to own anything in the 1930s is truly a dream come true.  Just to rise above the station one is in; to acquire some ownership and level of respect is what the America Dream is about.  To stoop and feel the dirt that belongs to them, to look around and see their property, to have a house of their own, to not be beholden to anyone--that is the dream that George recites repeatedly for Lennie like a mantra, the American dream.