George explains to Slim in Chapter 3 that he and Lennie were born in the same town and after Lennie's Aunt Clara died, Lennie just started to accompany him "out workin'." George adds that he and Lennie became used to each other so that it felt odd to be alone.
As further explanation to Slim, George tells Slim that he has witnessed many men who travel alone that have become anti-social and even mean because they are alienated and on the defensive:
"I seen the guys that go around on the ranches alone....They don't have no fun. After a long time they get mean. They get wantin' to fight all the time."
Slim agrees that the men are mean, and adds that these men withdraw and do not even talk to others.
This scene is a significant one as it underpins the theme of the fraternity of man that Steinbeck stresses in his novella set in the Depression. Steinbeck felt that in a fraternity men were stronger and could overcome oppressive conditions with the strength of others.
In chapter 3, George is talking to Slim about how he and Lennie met. George explains that they were born in the same town. George knew Lennie's Aunt Clara, who raised Lennie from the time he was a baby. George takes up for Lennie and explains that he's not dumb, just simple. George explains that when Lennie's Aunt died, George began to take care of Lennie. He says he has never thought of leaving Lennie because Lennie is such a loyal friend.