It's part of the life package that everyone gets once they arrive in the world.
But what is sadness? What purpose does it solve? At a glance, it would seem like sadness is just a negative reinforcement. That explanation seems very apt. It is also very wrong.
Robert Burton spent his life studying the science of sadness. In his classic 'Anatomy of Melancholy', he wrote "He that
I measure every
meet, grief I narrow, probing eyes. withI wonder if it weighs like mine has an easier size or-Emily Dickinson
Then again, so is everything else you feel. That doesn't make it any less real.
Inside Out does a good job of cutting through all of the years of trends of the 'believe in yourself and everything will happen for you' movement. Sometimes we don't get what we want, that doesn't mean we cannot be good at what we do. No matter how badly you want to achieve something, that doesn't mean it's going to happen, but maybe you can get close to it. It's perfectly alright to not feel happy all the time. Sadness has its place.
Taking away sadness won't make you happy, it will just lead to nothingness.