One reason why George trusts Slim is because Slim inspires that level of emotion amongst the other ranch hands. This is evident in Crooks who shows a certain level of deference to Slim in calling him a "real skinner." Steinbeck's opening description of Slim is one that tends to inspire trust in the other men on the ranch: "There was a gravity in his manner and a quiet so profound that all talk stopped when he spoke. His authority was so great that his word was taken on any subject, be it politics or love." This effect helps to explain how important Slim was perceived by others, and part of the reason why he inspires a sense of trust within George.
Another reason why George trusts Slim is that he is convinced that Slim will not betray his trust. When George tells Slim about what happened in Weed, George is quick enough to put aside his doubts in saying that "of course" Slim would not violate a sense of trust. Slim does not believe in duplicity and a sense of deception, which is part of the reason why he is able to put Curley in check, preventing him from endangering Lennie after his hand is broken. George understands that Slim occupies an importance on the ranch, reflective of how he inspires trust amongst everyone. It is for this reason that George trusts him.
From a technical standpoint, George has to be able to trust someone because his creator John Steinbeck wants to be able to convey information through him to the reader as well as to the audience when the book is turned into a play. Steinbeck called his book "a playable novel." It was written in such a way that it could be converted into a script for a stage play with ease. The play was produced on Broadway the same year that the book was published, which was in 1937. In a stage play, with a few exceptions such as Thornton Wilder's Our Town, almost everything has to be done with dialogue. Exposition is communicated to the audience through characters talking to each other. Steinbeck considered it important to describe that incident in Weed. He has George reveal some of it in the opening chapter when George is berating Lennie for molesting that girl. But he can't have George explaining everything to Lennie because, for one thing, there is only so much that Lennie can understand. Slim, on the other hand, is obviously very understanding and sympathetic, so George can tell him more about what happened. For example:
"Well, he seen this girl in a red dress. Dumb bastard like he is, he wants to touch ever'thing he likes. Just wants to feel it. So he reaches out to feel this red dress an' the girl lets out a squawk, and that gets Lennie all mixed up, and he holds on 'cause that's the only thing he can think to do....I was jus' a little bit off, and I heard all the yellin', so I comes running, an' by that time Lennie's so scared all he can think to do is jus' hold on. I socked him over the head with a fence picket to make him let go."
This is a flashback handled through dialogue. Why is the Weed incident so important? Because it was a precedent to what happened with Curley's wife in the barn. Together the two incidents foreshadow the probable future. Lennie doesn't understand his own impulses, and he lies to George consistently. George only knows what Lennie told him. When George sees the body of Curley's wife he will realize that Lennie is changing and that Lennie didn't tell him the whole truth when he said he just wanted to feel that red dress. Lennie is becoming a menace. George can't be with him every minute, and he is losing his control over this big, strong imbecile.
Steinbeck was considered one of the best dialogue writers of his time. Dialogue was Steinbeck's forte. He was compared to Ernest Hemingway, who was a master. Steinbeck's use of dialogue in his best novel, The Grapes of Wrath, is even better than in Of Mice and Men. Dashiell Hammett, a contemporary of those two authors, was also considered an outstanding dialogue writer, as can be seen in his best-known work, The Maltese Falcon.