Being sad in almost all cases is a choice (For Korea)

Emotions are all in your head.

They are obviously not physical and therefore should be controllable. 

If you believe that you won the lottery, regardless of if you did, you gain the joy that you did. 

If you live your life always assuming the best and not being crushed when you are wrong, you should always be happy?

Why don't you seek opportunities for joy constantly?

There are a plethora of available opportunities for joy that we dont seek.

So why don't we?

Because deep down anyone who is sad wants to be.

How long can you hold belief that you won the lottery before effect wears off?