The relationship between Lennie and George, in Steinbeck's novel Of Mice and Men, is not one which is completely understandable.
George is left with taking care of Lennie because of a promise he made to Lennie's aunt. George is capable, a solid thinker. Lennie,on the other hand, has a mental weakness. Lennie needs George to survive in a world filled with complications.
The reason that Lennie and George's relationship works can be based upon the old cliche "opposites attract." Lennie and George share only one thing--the male gender. Outside of that, the men are very different.
Although George may not admit it, he needs Lennie as much as Lennie needs him. Both men seem to encompass the best of both worlds: a sympathetic eye and a hard work ethic. The men need each other to become the whole necessary to find their dream. Neither Lennie nor George could survive without the other. When Lennie dies at the end,George is certainly a different man. With Lennie's death, a part of George dies as well.