Two psychiatrists are chatting together. What does this joke mean?

Two psychiatrists were chatting together. "What is the most difficult case you have encountered?" One asked. "I once met a patient who always believed that he had a rich uncle in South America and would leave him a lot of property, so he did nothing every day, just waiting for a letter informing him to collect the property." "What was the result?" "It took me eight years to cure him, but the hateful letter came."