If George no longer were in the company of Lennie, he would be like so many other Depression Era itinerant workers who travel alone from seasonal job to job, friendless and alone, with nothing to anticipate in the future.
In Chapter One, while George and Lennie camp out in the clearing, George describes what it is like to be a "bindle stiff":
"Guys like us, that work on ranches, are the loneliest guys in the world. They got no family. They don't belong no place. They come to a ranch an' work up a stake and then they go inta town and blow their stake, and the first thing you know they're pound-in' their tail on some other ranch. They ain't got nothing to look ahead to."
George also mentions here and in other situations that having a friend to travel with gives a man a sense of meaning and some worth, but if a man is alone, he has no one to look after him or care about him. With another man, George affirms that a man has someone by whom to "measure himself," as Crooks mentions. With Lennie, George always has someone to validate him, someone who listens to him, laughs with him, mourns with him--someone who keeps alive a dream of having a place of their own.
Without Lennie, George, at times, would have no one with whom to talk, no one with whom to laugh or cry, or simply to be with. Certainly, without a friend, there would be no one with whom to "measure things" and perceive whether experiences are beneficial or not. For George, a man's life has more meaning if he shares his thoughts and experiences.
Without Lennie, George would not have as much to worry about (or to complain about), but he would sense the separateness that others do, and he would be without hope and the strength and meaning of fraternity.