What is the psychological advantage of having a happiness set point? If one found a way to constantly be attached to this set point, what would the advantages be (other than perpetual happiness ^^)? How could a person become grounded in a perpetual state of happiness? What specifically would pull a person out of this state? How does a person get back in if they leave?
I think you've misunderstood the set point. It's a point of balance (or imbalance) of positive and negative affect toward which a person naturally gravitates. As such, there's not much need to do anything about leaving the set point except to wait / go on living and let time, neurochemistry, and daily life do the work.
Deliberate effort is more likely to attempt to increase positive affect beyond the set point. Thus the common struggle is generally against the effect of the "hedonic treadmill" pulling oneself back down to a more neutral or discontent set point after moments of hard-won happiness. Perpetual happiness is an elusive target for almost anyone, including positive psychologists. Most positive interventions have somewhat temporary effects (again, due largely to the true nature of the set point), though the small catalog of events that induce lasting changes to happiness is growing slowly through ongoing research.
From an evolutionary perspective, advantages to this aspect of human nature might include resilience, impulse control, and stabilization of one's identity.
- The hedonic set point pulls either way depending on the direction of deviation, and downward deviations are generally more threatening. The natural tendency to "bounce back" from depression, calm down from high alert, and otherwise recover from traumatic stress is the upside to the set point for most people (though some have set points that we classify as depressed, unfortunately).
- Excessive exuberance can be dangerous as well. It may relate to mania and disinhibition of impulses (I'd prefer to say excessive happiness sometimes results from these, but I'm not sure enough of it). If unusual circumstances increase happiness excessively (e.g., winning the lottery), dysregulated behavior with negative consequences may result. This may also be a mechanism of regression to the set point. Anxiety and discontent can also have motivating effects, whereas survival needs aren't always well-served by a state of bliss, especially when they entail hard work and worry for the future.
- Both individuals and groups may benefit from the affective stability of personality. This point is somewhat more speculative than the others on my part, but to motivate it, consider self-verification theory and the social undesirability of moodiness and anger. People prefer to be able to predict emotions in themselves and others, as the motivational impact of unexpected depression, unreliable energy, or uncontrollable anger is disruptive and sometimes dangerous. Of course, we live with this challenge anyway, but without a natural process for restoring emotional equilibrium, our vacillations could be more chaotic and troublesome than they already are.
Plenty of theoretical literature and research exists on this topic, and it's not hard to find. E.g., see the Wikipedia page you linked in your question. I listed one of those in my answer to this question:
· What exactly is meant when it is said that only you can make yourself happy?
· Lyubomirsky, S. (2008). The how of happiness: A scientific approach to getting the life you want. Penguin.