Obi's downfall is related to the many expensive duties he suddenly acquired after he returned home from England. Suddenly, he was responsible for providing for his parents' retirement income, paying his brother's school fees, repaying the Umuofia Progressive Union for the scholarship that allowed him to get his college education, and, last but not least, keeping up the social appearances and obligations of a man of his position. This last duty involves buying and maintaining a car, keeping a "boy" to cook and shop for him, and dressing appropriately for social functions.
Although Obi sees himself as an individual, his kinsmen maintain a group identity and incorporate Obi into their system of socialism and nepotism, creating a web of obligations and expectations that Obi finds difficult to escape.
Although Obi believes that his duties become his financial ruin, in reality it is his own poor financial planning that causes his downfall.