Some people might tell you that the purpose of life is to be happy.
Happiness is the emotion that informs you when you are making progress towards a goal. The problem is that progress happens in an inconsistent way.
In the book "Mastery" by George Leonard, the author describes the process of mastery as being non linear, if you would draw a graf of it would look something like this:
When you start practicing a new skill you are going to make a lot of mistakes, for a while there will be no perceivable, signs of improvement.
If you remain consistent with your practice you will eventually experience a short growth spurt, there is where you have a perception of progress that your body will recognize and communicate it to you with happiness.
If you are going to keep making progress, you have to keep challenging yourself in order to grow. When you do that there will be a small deep in your performance because of the increase challenge followed by another long plateo.
Most of your life will happen on a plateo where there is no percivebal progress so no happiness.
To say that happiness is something that should last, is like saying that one day you will eat something so good and high quality that the pleasure you get from it will last for the rest of your life.
Happiness is allways short lived and happens in episodes with large intervals of time in between.
There is also another problem about making happiness a priority, if happiness is the emotion associated with progress this doesn't necessarily means that it matters towards what you are making progress. Could be something constructive or destructive.
For example if a serial killer makes progress towards killing his next victim, he is probably going to be very happy doing so.
I think people who prioritize happiness might sell themselves a little short. It is not enough to be happy to have a good quality of life.
There is one positive emotion that is associated with the phenomena of sustainability and has the potential to last, that is love.
Love is what is sustaining people on the plateo and keeps them going despise the lack of immediat results.
But even love can't be the priority because sometimes people choose, disfunctional and unstable strategies to get love which is going to compromise other areas of their life, and make it difficult to experience other positive emotions.
This is why the priority should be on integrity. Which is the ability to choose strategies to meet your needs without compromising other areas of your life, and that's how you Will get the highest quality of life.