This is a big question.
In the book Of Mice and Men, the theme of friendship is central. What sets Lennie and George apart from every character in the book is that they are friends. It is an odd couple, but they are loyal to one another, and this quality makes them very attractive. Lennie is slow and strong; George is quick-tempered and clever. In their case, opposites attract. As for social status, they are on the same level.
In The Kite Runner, the theme of friendship and class is much more pronounced. Amir, the protagonist in the story, is friends with Hassan, who is a servant boy and, unbeknownst to them both, his half brother. The relationship between the boys is tense, even though they are friends, because Baba, Amir's father, has a hard time showing love to Amir on account of his guilt about not being able to show love to Hassan. All of this creates tension and confusion in Amir. Friendship, social class, and family relationships all come together.
When Hassan is sent away on account of Amir's lie, things become even messier. The rest of the story is about redemption. In the end, friendship emerges.