Well Carlson is a man with no friends, no hope or dreams and he is still clueless about companionship which tells the reader that Steinbeck wanted to end it as way of saying that the American dream was hopeless and unrealistic due to the idea that it was impossible to achieve even if you were of higher status or had more of an advantage to other workers (Slim etc...). You can link it to the theme of fate and loneliness and that the workers were never meant to find companionship as they all become lonely in the end (e.g candy's dog and the death of Lennie).
Another good interpretation could be that Carlson represents what George may turn out like without Lennie and that he may be fated to become like Carlson.