The novella does not tell us explicitly what happens to men who live and travel alone. However, we can take a few educated guesses.
First, people who live alone have a miserable life. We see this point most clearly in the character of Crooks. He has been alone on the ranch, because he is a black man. In a conversation with Lennie and Candy, this comes out. He says that being alone makes you crazy and eventually gets you sick. Here is what he says:
A guy needs somebody—to be near him.” He whined, “A guy goes nuts if he ain’t got nobody. Don’t make no difference who the guy is, long’s he’s with you. I tell ya,” he cried, “I tell ya a guy gets too lonely an’ he gets sick.”
If a person has companionship, then he has something beautiful. The sadness of the novella is that most people are alone. The only exception, for a time, is George and Lennie.
Second, it is not certain what happens to people who travel alone, but what we can say is that traveling alone is the norm. In a conversation between George and Slim, Slim says that not too many people travel together. The text says:
Slim looked through George and beyond him. “Ain’t many guys travel around together,” he mused. “I don’t know why. Maybe ever’body in the whole damn world is scared of each other.”
Based on this point, presumably men who travel alone live the rest of their lives alone in loneliness and sadness.