In the 3rd chapter in "Of Mice and Men," why do you think George tells Slim about what has happened in Weed?

With his "God-like eyes," the character Slim in "Of Mice and Men" is perceptive and kind.  Added to these qualities, Slim is subtle.  For example, when he and George return to the bunk house and speak about the puppies and Lennie,

Slim moved back slightly so the light was not on his face. 'Funnny how you an' him string along together.'  It was Slim's clam invitation to confidence.

Slim also taunts George into explaining about his relationship with Lennie by saying

'It jus'seems kinda funny a cuckoo like him and a smart little guy like you travelin' together.'

George explains, but then "wanted to talk" and Slim

neither encouraged not discouraged him.  He just sat back quiet and receptive.

So, when George realizes that Slim is truly listening to him, he confides in the man since he has spent so long without anyone to really talk with who is intelligent and interested.  As George continues to talk, Slim compliments Lennie as "a nice fella, and the lonely George who "began to lay out his solitaire hand"  begins to trust Slim and, thus, reveals what has happened in Weed with the girl in a red dress.

This section of "Of Mice and Men" underscores the terrible aloneness of the itinerant workers and man's basic need for brotherhood.  In their conversation, George mentions that it is bad for men to go around on the ranches alone:

That ain't no good.  They don't have no fun.  After a long time they get mena.  They get wantin' to fight all the time.

As Joseph Conrad wrote in his short story, "Meaning depends upon sharing"; George finds meaning in his life by talking with the kind Slim.