It's simple. Make $100K, live near people making $90K = rich. Make $100K, live near people making $110K = poor.
Here's another study to that effect: http://www.telegraph.co.uk/science/science-news/3315638/Rela...
"But the most the exciting finding was the influence of another factor: how the rival player was doing. Activation was at its highest for those players who got the right answer while their co-player got it wrong. But participants who got more money than their co-players showed much stronger activation in the "reward centre" than when both received the same amount."
This has a lot of interesting implications. For one, the classical homo-economimus model of humans as rational utility maximizers is further called into question. For two, it highlights the hedonic treadmill effect. No matter how much you earn, as long as you're comparing yourself to someone who makes more, you're unlikely to be satisfied.