The rest of us may take the question, with its obvious answer, as a rhetorical slap in such faces. Of course one would expect psychologists to go further and show just what is worth calling "happiness" and just how it is good; but they haven't. In the past, that's been the job of philosophers, who disagree completely about that as about all other questions. Psychologists, on the other hand, naturally concern themselves with problem people, not happy people; the literature corresponds. To most of them, happiness has not seemed worth studying in depth; when there are no problems to be solved, nobody gets paid to help solve problems. Concern for the secrets of happiness is left to pop writers passing themselves off as spiritual gurus whose perceived triviality grates all the more because it is so lucrative. But like the other prejudices of the mental-health profession, which is no longer ruled by Freudian or any other orthodoxy, that one is under increasingly credible attack. The general leading the charge is Professor Martin Seligman of the University of Pennsylvania, whose "positive psychology" movement is gathering steam. (Hat tip to Kooky Abuelita for the link to the London Sunday Times article.)
Disclaimer: my interest is not entirely objective. During my first year as a philosophy grad student at Penn (1980-81), I earned my keep by acting as "administrative fellow" for one of that campus' then-experimental "college houses," Van Pelt, where students lived with a few faculty who provided cultural enrichment and occasionally even moral guidance. The "faculty master" of that house was none other than Marty Seligman, then a rising star of his profession. Though I admired him for his brilliance and style, he and I did not have the coziest of relationships. I was a rather callow youth, and a tactlessly evangelical Catholic to boot, married to a salty older woman who was intimidated by nobody and didn't much care for him. None of that endeared me to him. We did our respective jobs and that was that. I never realized how pertinent his already-famous theory of "learned helplessness" would be to my recovery, twenty years later, from severe and chronic depression; nor of course did I or anybody else have any inkling that his interest in defining and promoting happiness had a sound scientific source in the research he had already been doing. But now that I think about it as a philosopher heavily influenced by Aristotle and Aquinas, Seligman's approach is as natural as can be. My personal interest in the matter has nothing to do with that.
In general, the bad can only be understood against the background of the good. Evil, understood as what is objectively undesirable either morally or otherwise, has no existence except as a corruption or falling away from good, which is ontically if not conceptually primary. The good is what is objectively desirable, and a living being's nature can be understood in terms of what good(s) it characteristically desires and pursues. Happiness, I would venture to say, consists in the reliable satisfaction of desire for the objectively good (as distinct from what superficially but falsely seems good). The great thing about Seligman's theory—which I urge people to read about in the linked article and follow up—is that it gives some scientific account of happiness thus understood in the case of human beings.
Beyond that, what excites me about Seligman's refreshing approach is that he is successfully overcoming mountains of prejudice in his profession. Academics in general and psychologists in particular are predisposed to smell triviality in any happiness shtik if not outright charlatanism. But he goes doggedly on, spreading his new gospel around the world from its formal base at Penn, gaining millions in grant money, producing as well as collating testable results. Nobody can get away with laughing at him in public anymore. (Save for faculty lounges, to be sure; no academic reputation emerges from them unscathed.) Even the woman I date, who is studying for a counseling degree, waxed enthusiastic about him before she ever knew I had known him. This guy is for real. That he is not another Thomas Moore or Deepak Chopra only adds to his luster in my book; people of any religious tradition can benefit from heeding his work without diluting their own tradition. There's nothing to dislike here, at least not in principle.
I intend to study this in much greater depth, and I recommend that others who care do the same. It will help me improve my life, and might even make Marty willing to talk with me again!