The speaker in the story confesses “…there must be men to whom I owe a dollar which I have forgotten. There may be a list of them. The more I think of it the less I like it…” and “I don’t count here men who may have lent me an odd dollar over a bridge table; and I am not thinking (indeed I am taking care not to think) of the man who lent me thirty cents…”
 
These statements clearly show that he doesn’t want to remember what he owes to others. But when the question comes ‘why’, it is difficult to find an exclusive reason for him. This is probably because he realises that if he starts remembering and listing all whom he owes a dollar or so, the list might get longer and longer and he might find it a real trouble to repay the debt. That is why he likes to remain forgetful of all those dollars.
 
Actually, the author wants to present the general human tendency in a satirical tone here that we all tend to forget what we owe others but remember vividly what others owe us.