why does George tell slim the truth about what happened in weed?

George finds a kindred spirit in Slim. George feels comfortable in explaining Lenny's limitations to him. George wants to confide how and why he helps Lenny. George knows that Slim will understand.

I think that George confides in Slim because he can trust slim. SLim is known as "the king of the Ranch", so I also think that this has something to do with it.