It often feels as if there is no such thing as a new friend in White Teeth. Friendships come with history, lots of history. You know that vague answer to the how-do-you-know-each-other question: "Oh, we go way back"? Yeah, it's like that. The significant friendships in this novel are all about lasting, standing the test of time. In this novel, friendship is an endurance sport, not a sprint.
Archie and Samad probably shouldn't be friends after they return home from WWII. After all, they have nothing in common and sometimes don't even seem to like each other.
Archie and Samad have a significant kind of friendship precisely because they seem to have nothing in common, and come from very different backgrounds.