Slim states, “I seen the guys that go around on the ranches alone. That ain’t no good. They don’t know how to have fun. After a long time they get mean” Why do you think the men get mean being alone?

This quote actually comes from George, but Slim agrees with the idea that men who travel alone "get mean." The reason that men like this become mean and nasty is simple: they are so consumed by loneliness and isolation that when they do make a human connection, they often vent the frustrations that are caused by this extreme loneliness.
We see lots of examples of this in the novel. Take Crooks, for instance, who delights in torturing Lennie by telling him that George might not come back from town. Crooks does not really mean to be so hurtful towards Lennie. What's happened is that he has become bitter as a result of years of isolation because of the color of his skin.
Similarly, Curley's wife also gets mean and nasty with Crooks in Chapter Four:

Well, you keep your place then, Nigger. I could get you strung up on a tree so easy it ain't even funny."

Again, Curley's wife is not nasty by nature. Instead, she has become mean and nasty because she is so fed up with her loneliness and isolation. Remember that not only is she the only woman on the ranch, she is actively ignored by Curley and the others. This has created feelings of isolation and powerlessness which she takes out on Crooks as soon as she gets the chance.
What Steinbeck is showing us, then, is that isolation and loneliness can bring out the very worst human characteristics. It is for this reason that we need friendship in our lives, as demonstrated through the characters of George and Lennie.