It's rude to be antisocial at work.
John has been suffering in his personal life but still has to work with others.
John just wants to get through his work day and have it over with.
John keeps his personal life out of the workplace and tries to stay busy at work so the day goes by faster.
John avoids people all day at work because of his sour mood about his personal life hoping it'll make the day easier for him.
John's co-workers are undisturbed by John's problems which are not theirs, and they work flow goes smoothly for the day.
John's co-workers find him difficult to work with and John becomes a bottleneck for everybody's work flow.