There are many rational explanations for why money doesn't buy happiness, but that rationality is conspicuously absent when most people go shopping. The problem is best elucidated by the psychological term hedonic treadmill: "The tendency of a person to remain at a relatively stable level of happiness [or unhappiness] despite changes in fortune or the achievement of major goals." Or described another way, "the pursuit of happiness to a person on a treadmill, who has to keep working just to stay in the same place."
The literature surrounding this term suggests that throughout most of human history, we've had very few possessions. But since we're instinctively influenced by a hunter/gatherer mindset, our modern incarnation quickly defaults to believing that more is better. With the advent of currency and an ever increasing array of objects for sale, once we've satisfied our essential needs, we emotionally shift to buying increasing amounts of stuff, expecting that it will make, or keep, us happy. The obvious flaw here is that it typically doesn't.
What usually happens is that we see something for sale and buy it in the belief that it will continue to make us as happy as we are at the point of purchase. When the initial buying euphoria wears off, we decide that the object didn't live up to our expectations and then go on to repeat the process. If this was the end-of-story, the conclusion that money can't or doesn't buy happiness might be valid. But maybe it isn't.
Consider the currently ubiquitous example of buying large high-def flat screen TVs. Your friends have them, you want one, and you enthusiastically go out and buy one. After a few weeks time, depending upon how much you watch, the new dazzling high def screen in your living room reverts back to its role as 'the TV'. It seems that the degree to which we derive sustained pleasure from purchases has something to do with how often we interact with them.
I purchased an inexpensive antique car many years ago. The antique license plate places limitations on its use -- but also reduces insurance to only 50 bucks/year -- and I typically only drive it on weekends in the summer. I get the same charge out of driving that car today as I did the day I bought it. Other purchases that keep on giving may include fine jewelry, a musical instrument, a special garment, a great fishing rod, unique cookware, vintage port or even buying more time with loved ones by paying someone else to do your chores.
The key to successfully buying happiness might well lie in the judicious utility of the object or service bought. And while merging emotional impulse with rationale analysis the next time you go shopping, consider too that one of the best ways to feel happy is to make someone else happy.