Steinbeck gave George and Lennie many contrasting characteristics in order to make it easy for the reader to visualize them and tell them apart. Their contrasting physical and psychological traits are pointed out by the uneducated but very intelligent character called Slim.
"It jus' seems kinda funny a cuckoo like him and a smart little guy like you travelin' together."
George is a smart little guy. Lennie is a powerful giant but mentally incompetent. Lennie has to depend on George to tell him what to do, but he resents some of the orders he gets, especially regarding his interest in petting little animals. As a result, Lennie has developed a tendency to tell lies. He probably doesn't know the difference between lies and truth. Both men share a dream of owning their own small farm. Even Lennie is smart enough to realize that it is a dog's life working from sun-up to sundown, sleeping on a straw mattress, gobbling down cheap, ill-prepared food before it is all gone, and having nothing to show for years of toil but a broken and worn-out body like those of Candy and Crooks.
George feels that Lennie is a burden. At the same time, however, he realizes he is getting something out of the relationship. Being able to share dreams and secrets with another person has a humanizing effect on both of them.
"I ain't got no people," George said. "I seen the guys that go around on the ranches alone. That ain't no good. They don't have no fun. After a long time they get mean. They get wantin' to fight all the time."
Most of the men feel instinctively that they are competing for their very existence. Being loners makes them mean. George and Lennie are different because they cooperate. They are real friends. Steinbeck believed that the world would be a better place if only people would cooperate instead of competing and fighting.
Both Curley and Carlson are examples of men who see the world as a place in which it is every man for himself. Curley is always looking for a fight. Carlson is not pugnacious, but he is the only one of the bindlestiffs who owns a gun. No doubt he feels that he needs it, living the kind of life he does. He sleeps in the open and travels on freight trains with desperate men who would cut his throat in his sleep if they thought he had anything in his pockets worth taking.
George is a "little guy." He must be well aware of the fact that having a powerful friend like Lennie is an advantage in his world. George is afraid he might have to fight Curley himself.
"Ya know, Lennie, I'm scared I'm gonna tangle with that bastard myself. I hate his guts."
This is a situation in which George does not have to get into a fight which he would probably lose. He and Curley are the same size, but Curley is a semi-professional boxer. Candy tells George:
"Curley's pretty handy. He done quite a bit in the ring. He's a lightweight, and he's handy."
But Curley makes the mistake of picking on Lennie instead of George and gets his hand so badly mangled that he will never fight again. This incident demonstrates one of the benefits George derives from having Lennie as friend and companion. They have a symbiotic relationship. George provides the brains and Lennie provides the brawn. Lennie could never have his own farm without George, but George could never have his own farm without Lennie.
George and Lennie are both hard-working, peace-loving men just trying to survive in a hostile world ruled by greed, fear, and the never-ending battle for survival.