When George and Lennie show up at the ranch towards the beginning of the book Of Mice and Men, Curley, the boss's son, eventually questions them. He first looks them over in a searching and unfriendly way:
"His eyes passed over the new men and he stopped. He glanced coldly at George and then at Lennie. His arms gradually bent at the elbows and his hands closed into fists. He stiffened and went into a slight crouch. His glance was at once calculating and pugnacious."
It is clear that Curley is combative and judgmental; he immediately balls his hands into fists and looks at the newcomers is a belligerent way. When Curley asks if they are the guys the boss was waiting for, George replies: “We travel together." Curley responds, “Oh, so it’s that way.” Curley is clearly suspicious of George and Lennie, as most ranch hands don't travel with other people. They are generally loners, so two men traveling together is an oddity. In addition, perhaps he suspects that they are having a romantic relationship, as men on the ranch usually don't attach themselves to other men and do not make many friends, given their migratory lifestyle. Finally, perhaps Curley is just generally suspicious of other men, as he has just married and thinks his attractive wife is eyeing other men.
When George tells Curley that he and Lennie travel together, Curley responds with the sardonic comment, "Oh, so it's that way". Like his father, Curley is suspicious of George's motives, insinuating that perhaps George is taking advantage of the less intelligent Lennie in some way, using him by having him work, then taking his wages, for example. Curley's father expresses the feeling behind Curley's suspicions more clearly by observing that ordinarily a man doesn't "take so much trouble for another guy". In the world of itinerant ranch workers, men look out for themeselves first, and usually avoid making lasting connections with other men.