George shows his sense of responsibility towards Lennie when he tells Curley,
I told his old lady I'd take care of him. He got kicked in the head by a horse when he was a kid. He's awright.
Lennie admits he is responsible for misdeeds when he goes to the Salinas pool after killing Curley's wife. He is wracked with guilt as he has an imaginary, confessional conversation with his Aunt Clara:
I know, Aunt Clara, ma'am. I'll go right off in the hills an' I'll fin' a cave an' I'll live there so I won't be no more trouble to George.
As he takes responsibility, Lennie is willing, at least for the moment, to sacrifice himself to help out George.
Lennie speaks with raw honesty to George when he expresses his intuitive fear that the ranch is a dangerous place for them to be:
I don' like this place, George. This ain't no good place. I wanna get outa here.
George is equally honest when he agrees with Lennie, saying,
"No, I don't like it .... For two bits I'd shove out of here."
He is honest, too, at the end, when he tells Lennie right before he shoots him that he has never really, deep down, been angry at Lennie:
"No," said George. "No, Lennie. I ain't mad. I never been mad, an' I ain't now. That's a thing I want ya to know."
Understanding can mean both factual and emotional understanding. In the quote below, Lennie shows a factual understanding of how George expects him to behave when he properly answers George's question about how he is supposed to act when they arrive at the ranch:
Lennie stopped chewing and swallowed. His face was concentrated. "I ain't gonna .... say a word."
George praises him for his understanding.
George shows an emotional and intuitive understanding in knowing what Lennie needs to hear to feel comforted and at peace before he shoots him:
You .... an' me. Ever'body gonna be nice to you. Ain't gonna be no more trouble. Nobody gon-na hurt nobody nor steal from 'em.
Although they make mistakes and are sometimes deceptive, George and Lennie reveal themselves at heart to be decent, sympathetic people.