Slim doesn't share in the other men's dreams as he is presented as being on a level apart from most other people. He is infinitely wise and knowledgeable about life. He is a philosopher and realist, not a dreamer. But although he holds himself somewhat apart from others, he does not despise them. Quite the contrary; he accepts and understands other people better than anyone else.
His hatchet face was ageless. He might have been thirty-five or fifty. His ear heard more than was said to him, and his slow speech had overtones not of thought, but of understanding beyond thought. (chapter 2)
He recognises the good and evil in other people, immediately realising that Lennie is a good-hearted type despite his slowness.
In his wisdom, Slim also encourages others to confide in him, and he is generally looked up to. He is the only one at the ranch that George really opens up to, and he provides support for George at the end over the shooting of Lennie. The alternative - that Lennie be confined to a mental asylum or hunted down by the bloodthirsty Curley - Slim rejects as too awful to contemplate. He helps to provide a less conventional, but ultimately more merciful solution.
Slim is an example of the calm, all-seeing, all-understanding figure who recurs several times in Steinbeck's work, for instance Doc in Cannery Row and Casey in The Grapes of Wrath.