It appears to me that we love what we can’t/don’t have, and we long for things we are short of. I have come across examples where people are so obsessed with achieving what they don’t have that they forget to appreciate what they have!
And there, you’ve the very foundation of our sadness in one word –EXPECTATIONS!
If expectations are met, life’s splendid and colorful. Whenever turning expectations to reality fails, the foundation for sadness is laid.