Mark Manson shares a theory from a book on happiness:
Gilbert’s theory is that we each have a “psychological immune system,” basically a bullshit generator where our minds explain away our past experiences, our future projections and our current situations in such a way that we always maintain a baseline level of mild happiness. And it’s when this “immune system” fails that we fall into prolonged depression and/or existential crises.
A direct quote from that book's author:
We treat our future selves as though they were our children, spending most of the hours of most of our days constructing tomorrows that we hope will make them happy… But our temporal progeny are often thankless. We toil and sweat to give them just what we think they will like, and they quit their jobs, grow their hair, move to or from San Francisco, and wonder how we could ever have been stupid enough to think they’d like that. We fail to achieve the accolades and rewards that we consider crucial to their well-being, and they end up thanking God that things didn’t work out according to our shortsighted, misguided plan.
- Daniel Gilbert, Stumbling on Happiness