"It ain't so funny, him an' me goin' aroun' together," George said at last. "Him and me was both born in Auburn. I knowed his Aunt Clara. She took him when he was a baby and raised him up. When his Aunt Clara died, Lennie just come along with me out workin'."
While many of the men on the ranch where George and Lennie go to work think it's odd that the two men travel together, George explains it simply: He knew Lennie's aunt; she passed away, so now he takes care of Lennie. And to George, that's that. George understands Lennie isn't capable of taking care of himself:
"If you don' want me I can go off in the hills an' find a cave. I can go away any time."
"No—look! I was jus' foolin', Lennie. 'Cause I want you to stay with me."
Lennie is physically strong yet mentally weak; while George is, at least to some degree, his opposite. They are, in Freud's terms, id and superego of the same person--Lennie is all instincts and drives, while George is all judgment and morality. And to George, he simply has a moral obligation to watch out for Lennie. Steinbeck portrays this idea so unquestioningly in these two characters that when others question why George would bother with Lennie, they themselves seem to be the ones doing some morally obscene thing. Why would a decent human being NOT care for another who can not care for him/herself?