At first Crooks jeers at Lennie, evidently because he is intellectually limited, but the real "stab" he makes is when he suggests that George could go away and never come back. Lennie can't make the difference between a hypothesis and a fact and thinks Crooks is telling him that George will indeed not return that evening or ever. He begins to panic at the thought, and then it is Crooks turn to calm him down.
Up to this point, Crooks has not considered anyone's misfortune other than his own, and suddenly he sees another person's vulnerability and despair. He softens up to Lennie, and even lets his own guard down as he talks honestly to Lennie about his own solitude.