Butters put it better than anyone
I would say sadness is a good thing, because if you've never been sad you are likely empathetic. Everyone has rough points in their life. But, as others previously stated, too much sadness can be detrimental to your health. As hard as it is to believe, sadness is actually a good thing provided you don't feel sad all the time. It's proof that you're still alive, and that's quite an accomplishment if you think about it