What does "like a boil that can never be cured as long as it is covered up but must be opened with all its pus-flowing ugliness" mean?

"the tune the he might never know again." this part build anticipation. dexter got extremely happy by the piano tune. he could see what he was living at that moment in his life throught this feeling. the reader might want to learn more about his happy life. in fact, he called it a state of   'ecstasy'. the fact that he had got in a mood of intense appreciation and that everything about him was extremely bright and glamorous might lead the reader to feel that this state of happiness may not last long. the two parts from the excerpt that exemplify the anticipation are: " the sound of the "; " it was a know again."