It's known as the "Hedonic Treadmill," and I have posted about it here. People get a small rush when they acquire something new, and mistakenly believe that larger/better acquisitions will result in better/longer good feelings. Really, it's the novelty of the thing that makes us feel that way, and novelty inevitably fades. The thing is, many people never realize this and end up chasing that feeling for their entire lives, hence: eternally walking on the Hedonic Treadmill that takes them nowhere.

Some people are less susceptible to this than others. If you don't have a particularly addictive personality, or if you are not in the fog of consumerism as a positive thing, you will probably not struggle with this as much as someone who has one or both of those traits.