Don't say i don't have reply in your post. .... 
Though my post will seem like LPPL or worthless. But it's my 2
cents.
Happiness is subjective.
Being optimistic and pessimistic could be developed.
Pessimism and optimissim are perceptions.
Happiness are (mainly) emotions, expressions.
It is possible.
It happens to every single human at different phrase of life.
The question is : For how long can one hold?
As ageing and growth happens, he will realise he/she lives in
contridiction.
This struggle may be Mild to some, serious to others.
Ways to overcome :
::: Acceptance
::: Change
Either ways are good at the appropriate time and situation.
You can fine tune perceptions/perspective, you can't
do the same with happiness/being happy.
Cheers
