The word sadness originally meant fullness," to be filled to the brim with some intensity of experience. It's not about despair, or distraction, or controlling how you're supposed to feel, it's about awareness. Setting the focus to infinity and taking it all in, joy and grief all at once; feeling the world as it is, the word as it could be. The unknown and the unknowable, closeness and distance and trust, and the passage of time. And all the others around you who are each going through the same thing.
The Romans called it lacrimae rerum, the "tears of things." We call them obscure sorrows.
"I read the dictionary. I thought it was a poem about everything."
Ancient Greek καιρός (kairos), a sublime or opportune moment + σκλήρωσις (sklḗrōsis), hardening. Pronounced “kahy-roh-skluh-roh-sis.”