Some internal locus of control and having positive personality traits are the two most significant factors affecting one’s subjective well-being otherwise life is a Hedonic Treadmill.

Set Point of happiness is in your hands. “A true saying it is, Desire hath no rest, is infinite in itself, endless, and as one calls it, a perpetual rack, or horse-mill”. as St. Augustine, cited in Robert Burton’s Anatomy of Melancholy.

In a very long-term view, everybody will be happy as happiness will be genetically guaranteed till then happiness is impossible to achieve and all that we can do is to try to avoid pain as much as possible. In a Stoic point of view they aren’t all that bad