Happiness is as Nietzsche said, a straight line, a goal, a yes and a no.
Happiness appears instantly when we formulate a meaningful goal for ourselves. As soon as the goal, often figured as a desire, appears in our minds we feel a surge of happiness (or at least I do; I am speaking for myself here, so I will just do that). Then this surge of happiness will either continue or it will abate, depending on if I act in pursuit of that goal/desire.
In the absence of meaningful goals or desires, I find that my happiness sort of dissipates. Sometimes this dissipation is slow, other times it is sudden. But it is very important to have goals and desires that are meaningful, and to have enough of these so that when one is achieved it does not leave a vacuum.
Therefore, happiness seems to be a psychological response recognition of a metaphysical factual state, and plays itself out phenomenologically for lack of a better word. I relate feelings to color vision as an analogy to explain how these sort of phenomenological things work: we see different colors because these different colors are the direct representation of different wavelengths of visible light spectrum, and the individual colors themselves do not have a reason for being that color which they are other than the reason of how each such color is simply the immediate recognition of a difference, namely the difference in wavelengths of light.
There is no deeper mystery or "qualia" in colors, they are simply this immediate recognition-awareness of these wavelength differences as such, and nothing besides. Natural selection had to come up with a way for the organism to achieve this immediate recognition-awareness, in us humans the way this was achieves was simply in the various colors that we see. There is no reason why blue is blue and not something else, other than the fact that we just needed a way to differentiate these wavelengths for the purposes of navigating and understanding the environment as visually accessible to us.
Similarly, feelings like pleasure, pain, happiness, despair, these have the same phenomenological format; these feelings are immediate recognitions-awarenesses of something. What is this 'something'? It varies in the case of each feeling. We can individually analyze each feeling philosophically and personally-subjectively in order to figure out what it is that is being recognized-made aware of to us. In the case of happiness, I think what we are being given into immediate recognition-awareness is something like being in a state of meaningful goal-pursuit and desire-achievement. The goal and the desire then derive from deeper-held values and valuations, some of these intrinsic to ourselves at the biological and existential levels and others acquired and become meaningful to us for other more circumstantial reasons. And at the very bottom, all acquired or circumstantial reasons ultimately become intrinsic anyway, otherwise they would never have been able to arise as goals/desires in the first place.