How did George previously treat Lennie, and why did that change?

George says that when he first began traveling with Lennie he found it funny to play pranks on him. One day he ordered Lennie to jump in a river even though he couldn't swim and Lennie unthinkingly obeyed. After George fished him out, Lennie was completely grateful, having forgotten that George had ordered him into the river in the first place. After this episode, George decided against having fun at Lennie's expense.

How do they treat lennie on chapter 1-3 on the book "mice of men"

Why do u think he author repeats the story of what happen in weed?