Hughes describes jazz as "the tom-tom of joy and laughter, and pain swallowed in a smile." How is this phrase a paradox?

Joy and laughter versus pain are very different things, and some would argue they are complete opposites. When listening to jazz, feeling a mix of joy and laughter "and pain swallowed in a smile" would suggest that even though it's a happy feeling, there are darker and more sinister feelings underlying. This phrase is a paradox because pain and joy are opposites, yet they are used in the same sentence to describe the same thing.