Writer Eckhart Tolle speaks of "negative remnants of negative emotions" which accumulate in our body as a form of energy he calls the pain-body, "only active for a certain percentage of the time" but which "seeks unhappiness and unpleasant experiences, and more negativity because it feeds on those things." Perhaps this is another way to understand "set point" and how it can be modified through greater awareness. One of his articles on my site is Eckhart Tolle on Our Very Unhappy Entity: The Pain-body http://talentdevelop.com/articles/OVUETP.html

Your post interested me as I have been pondering about that question myself, on how to have lasting happiness. On just expressing my own perspective on this topic... I believe that lasting happiness and inner peace is something that has to come from within the individual rather than from external circumstances, because circumstances do change and the happiness that depends on them very fragile. And when one has too much of a good thing, somehow it stops bringing happiness or joy. Personally, I think that personal development is one of the key to lasting happiness that would prevail despite changing life circumstances. People often have a transformed outlook an approach to life after a life-changing event. Companionship or having close ties seems to be important too. After all, we all have heard and seen of miserable millionaires and individuals who are happy and fulfilled despite not having much.

I personally thinks about how to achieve happiness frequently. In fact, my concern is also geared towards what happiness should really be actually.

Laughter, feelings of euphoria are considered happy feelings. What about when you are feeling neutral, which happens often also. Does that mean you are not happy? Or does it mean that you are happy if you are at least neutral in your feelings right now.

external factors (including one's health and genes) : 60
controllable factors : 40

its quite difficult to really give a number though.
I feel that both your body and you environment has a larger control over how happy you are , even if you have heard many advices of how to be happy from books and famous people.

Its not that easy to achieve happiness all the time. That said.. I think most people are contended and happy quite frequently also. Only some of them will break down because they have met with bad circumstances in life and they do not have much wisdom to overcome it and remain happy.

Personal development and inner growth have a lot to do with our happiness. We need to learn to take the positive out of every situation. Although, sometimes that is a personal transformation that some have to come to and that can take some time to change their way of thinking and asking for what they really want.