What motivates all four characters, lennie, george, candy, and crooks to get their own farm?

Just to clarify, getting their own farm was only a faraway dream that they had, which didn't come true in any part of the book (spoiler: Lennie died in the end). In simpler words, their motivation came from George and Lennie's friendship so that they could lead a "better life" so that Lennie could have his farm with lots of rabbits. Candy's motivation-- his influence, came from the optimistic views of Lennie and George and Crook's influence came from Lennie, and how he told Crooks that they had everything planned out-- it shows Crook's drastic change in heart from pessimistic to optimistic. Because Crooks is black, he's treated unfairly (setting of the book is 1900s, I believe) and for once, he has hope to get out of the farm and start a new life with George, Lennie, and Crooks.