In the beginning of the book George and Lennie are talking about their dreams of owning a farm, and a snake appears. George and Lennie's dream symbolizes the American dream, and the snake symbolizes evil, and foreshadows that their plan won't work out. This scene shows how the common American man tries to achieve his goals, and how they sometimes end due to problems.
George, Lennie and Candy begin working together for the same dream. Candy symbolizes how years of work and effort can just barely get you towards your dream, and this scene shows how teamwork can help people achieve their goals even faster.
Lennie panics and accidently strangles Curley's wife to death. Curley's wife's death symbolizes how most dreams and plans fail due to hard obstacles. This scene shows how a single mistake can cause everything to fail.
At the end of the book, Lennie is killed by George. Lennie's death is symbolism for the death of their dream. This scene shows how the American dream can take effort, and it can either succeed or collapse.