People are very good at acclimatization, of getting acclimated to whatever situation they find themselves. It’s one of the fundamental consequences of the human condition. You know what I’m talking about: when your life’s situation changes, even in drastic ways, you get used to it relatively quickly.
This makes sense from a survivalist point of view. The quicker a hunter-gatherer can adapt to new animal migration patterns, types of predators, invasions of non-native flora and fauna, the better chance that she’ll propagate her genes. I think there are parallels from modern life, but I’ll leave those as an exercise to the reader.
Quick adaptation explains why people who get a pay raise (or a pay cut) experience a temporary increase (or decrease) in happiness before returning to their steady-state. Some people are going to enjoy their lives no matter what. Some people are going to be miserable even if they’re millionaires. There are a lot of depressed upper-middle class suburbanites. There are also a lot of happy slum-dwelling Kenyans.
But because one’s perspective often gets entrenched at whatever you’re used to, it’s a useful exercise to shift your focus from time to time. Covey’s Seven Habits is a pretty good introduction to that sort of thing. It’s my opinion that perspective is one of the scarcest resources in the ‘verse.