MuchMoreThanMuscle saidState of mind is really all that matters. Being happy is better than being miserable and if you can be happy even though you know you're only going to live for two years, two months, two weeks or two days it beats feeling terrified, sad or other less favorable feelings.
It really depends. Many artist depend on feelings other than happiness to do their best work. I know I'm at my best as a writer when I allow myself to feel a range of emotions. The push to make happiness a moral good feels oppressive at times. Let's not forget that the pursuit of happiness can sometimes be anxiety filled quest that leads some to become less content. I'm happy their is research allowing people to defend themselves from the joy police.
I was listening to a podcast yesterday where a German reasercher spoke about her research that showed that simply thinking positively about something lowers the likelihoods you will achieve. Apparently simple positive thinking blinds us to the obstacles that stand between us and what we want. Positive thinking on its own is detrimental if not combined with an honest assement of the obstacles we must overcome. She only suggest just using positive thinking as a form of pleasure for task that you cannot or do not truly want to achieve.