Because “happiness” is a mental construct. It’s not a physical thing, like a tree or a cloud, therefore answering this question is about the same as answering this other one: why is that, that in a random group of people, half of them are shorter? or half of them are poorer?
Well, the answer is simple: because it HAS to BE that way.
As Alan Watts said: you don’t encounter people with fronts but no backs, or coins with heads but no tails. They go in pairs.
So that what you call “happiness” is half of your experiences, and “sadness” is the other half. No matter how hard you try to fix problems and move things from sadness to happiness, there will always be a line splitting your experience into those 2 values, in half. Why? Because YOU do IT.
So the next question is: how can you STOP doing it? Well, first of all, you have to realize that, it’s not because you do it now, that you have to do it always. You can always decide to STOP, it’s up to you, it has always been.
Seeing happiness implies seeing sadness. Seeing richness implies seeing poorness. Seeing health implies seeing sickness. And feeling pleasure implies feeling pain.
All those are mental construct. You do them because they are USEFUL to you. And the minute you will stop believing into their usefulness, at that minute, you will be FREE from them.
Also published at: https://www.quora.com/Why-cant-there-just-be-happiness/answer/Thomas-Godart