Why are people happy?

Why are people happy? Or maybe a better question is, “How are people happy? Is there an objective standard by which we can identify happiness? Is there something built into the soul of man that, when met/fulfilled, will result in happiness? Or is it simply a subjective state based on what each individual decides is the goal? Should happiness even be a goal for the human? Could it be that happiness is simply a subjective state, defined by the individual that is simply a manifestation of selfishness? Are we being self-centered when we strive to be happy, as this state usually has to do with our personal comfort, or content state of mind?

I see people that, judging by the Orthodox standard of truth, should not be happy, but by all general appearances, they are. It cannot be that the presence of sin takes away the possibility of happiness, for we are all sinners, yet some people appear to be happy. It cannot be that our failure to recognize the particular sin is a factor, for there are many people who do not even recognize the reality of sin, i.e., atheists. If the onus falls upon us, that is, if we recognize the sin, we won’t be happy until that sin is dealt with, then every Christian would be perpetually unhappy and, by default, the atheist would be the genuinely happy one.