as i get older, i see there is a purpose for happiness and for sadness. when you’re happy, you have no reason to question it, you are comfortable happy. when you’re unhappy (mental, physical, etc) you’re uncomfortable, which means you tend to narrow your focus to the cause of the pain and then start to construct a plan to end it and get back to happiness. 
it’s in this discomfort we learn far more than we would care to when in comfort.