Life is originally a drama.
The moments we want to keep and the time we want to miss slip away quietly in the development of this complicated drama. In the end, there are many things that we feel are impressive, but there are very few things that can really explain why. So, we call this sadness.
When we look back, we stand in time and watch the vast swaths of sadness and happiness rushing past, but there is nothing we can do - we are just passing by.