I heard about the concept of the "hedonic treadmill" a few years ago and find myself frequently using it as a metaphor in conversation, but never being able to explain it adequately. I just read a good explanation in the excellent book NutureShock:
Back in 1971, two scholars…described the human condition as a "hedonic treadmill". Essentially, we have to keep working hard just to stay in the same relative place in society. Even when our situation improves, the sense of achievement is only temporary, because our hedonistic desires and expectations rise at the same rate as our circumstances. Brickman and Campbell noted that lottery winners are not any happier, long-term, than non-winners, and paraplegics are not less happy than those us with all our limbs. They argued that this plight was inescapable, due to our neural wiring. Our brains are designed to notice novel stimuli, and tune out everyday, predictable stimuli. What we really notice, and are affected by, are relative and recent changes. As soon as those become static, we return to a baseline level of well-being.
That we are so adaptive can be a good thing. When life falls apart, we'll soon get used to it - such changes in circumstance don't have to become incapacitating. But when our lives are blessed, and things are going well, there seems something morally decrepit in how we so easily overlook how good we have it.The authors go on to point out subsequent research which has revealed flaws and subtleties in this theory, so I wouldn't think of this as established science: it's more like an interesting way to think about what really makes you fundamentally happy vs. what is mere novelty. For guidance on that front I recommend Man's Search for Meaning.