If happiness is so wonderful to experience, it'd be natural to completely shun away from its polar opposite, sorrow. Depression itself can be classified as a mental illness. People prefer happy endings over sad ones. Nobody comes up with a book titled "How to be sadder in X simple steps!" and sells millions of copies... or at least I don't know of such an instance yet. The proportion of people that like to indulge in feelings of sadness or anger are seen as "abnormal"; look how often the emo/goth kids get teased!
Or are we also are foolish for choosing to not even look sadness in the eye? Why are we so afraid to be sad or angry? Most of us have been conditioned to "look on the bright side of life" and "smile even when it hurts". Is this correct behaviour?