WHY DO WE EXPERIENCE HAPPINESS?
If everything in nature has a purpose or function, or is the residue of a previous functioning system, what is the function of happiness? Strictly speaking we do not have to be happy in order to survive. In the eyes of nature, survival and reproduction are the two only goals. However, happiness does have a crucial role otherwise it would not exist. As we have previously discusses feelings are feelers for our environment, they can either alarm us or reaffirm us. We experience reaffirming feelings as good. Happiness is the signal that things are in order, functioning, and in balance.
Balance is the state in which things are just right within our body and our environment in that moment. For example, feeling good from a bit of sunlight on the your face indicates that it is just the right amount of sun, you are not getting burnt to a crisp or deprived from not enough of sun. It feels good because your body needs a bit of sun in order to keep homeostasis in its complex functioning.
With an ever-changing environment and many complex requirements to maintain homeostasis in the body, balance is also in a constant state of movement and change. (Think here of a spinning top, it stays up right only in movement.) If you stay all day in the sun, you might get too much of it, if you only got out for tow minutes it might be too little for your body. Time is a crucial aspect of balance.
Now if being in balance is what makes feelings of happiness emerge and balance is ever changing due to environmental changes, than it is no wonder that happiness never lasts. It is not a permanent state we can achieve, it is meerly a temporary indicator that our body and environment are in harmony.
On the other hand when our body and the environment are not in balance, we experience unpleasant feelings. And ones that threaten our survival go as far as being alarming. These alarming and unpleasant feelings are generally much louder and better at catching our attention because they come higher on nature's survival priority list than slight adjustments in maintaining homeostasis. However, happiness also needs our attention, otherwise it just wafts away like soap bubbles and the information gathered from the experience, the dataset collected is lost. This collection of data, unlike the alarming feelings, does not require an immediate reaction. Happiness just wants us to keep on observing to catch any imbalance as soon as possible. We naturally want to keep in a state of balance. Happiness is normal!
The more we observe our own steady-state of functioning the more we learn about it, the easier it becomes to do repairs should anything break. That is the purpose of happiness. It empowers us to live a good life. (And what could be better for our survival, nature's end goal, than that!)