Recently in many areas, it has been discussed on happiness or well-being. People ask, what is happiness, what are the factors that influence the happiness of people, how must live people to be happy, and what conditions must be provided for our happy life. Until now, various theories about happiness have been developed. For example, hedonistic theory, eudaimonia / prudential theory, comparison theory, self-determination theory, subjective and objective theory, emotional and cognitive theory, set-point theory of happiness, authentic theory, etc. Accordingly the concept of happiness is understood in various ways, such as pleasure, excitement, joy, contentment, peace, fulfillment, serenity, utility, comfort, well-being, quality of life, etc.. Before we answer the question of how we have to live to be happy, and what are the conditions of the happy life, we must first answer the questions, such as what is happiness and what is the nature of happiness. The former is a theoretical question of happiness, while the latter is a definitional question of happiness. And the former depends on the latter. But the diversity of the concept of happiness leads also to the production of various strategies of happiness, which often conflict each other. In this context a reexamination of the concept of happiness is necessary. In this paper, I discuss the concept of happiness, not on the mental dimension, but on the existential dimension, as a sort of existential feeling, i.e. as a sort of existential sense, which is a byproduct of life activity/activation. But in this definition of happiness two concepts play an important role; the non-equilibrium system and the set-point. What I mean with the former is a system which is self-closed and maintained by himself, whereas his systemic characteristic distinguishes from his environment. And what I mean with the latter is a certain level of non-equilibrium state, maintaining of which is necessary for the sustaining of the system. I apply these concepts to the existential world/system, and call each of it existential non-equilibrium and existential set-point. The set-point of existential system is a point, at which people are thriving. In this sense it is a normative one. But real existential status of people can correspond to it or not. And the case of non-corresponding divides into two cases; the case in which existential status of people lies below of it and the case in which it lies above it. Gaps between the two can be disappeared again by upward adjustment or improvement of the existential status of the people, by downward adjustment of the existential set-point and by sharing/distribution of the spare of existential resources with/unto environment like another poor people/animal. And in the case of resetting of existential set-point to the higher level, the gap between it and existential status of people can be disappeared by an another improvement of existential status of the people. Whereas the correspondence between the two is understood as state of happiness, i.e. of existential flourishing, various kinds of happiness can be defined according to the ways of the filling of gaps between the two. I close the paper with some comments on the matter of the theoretical question of happiness such as question about the factors that affect happiness, and question about how can we live to be happy. Even though these issues are beyond the scope of this paper, my discussion of happiness gives at least some important hints concerning with the question, how we can handle them.