Do you remember the first time you got behind the wheel of a brand new car? There’s nothing like it. You feel deliciously conspicuous and exhilarated, almost high on that new car smell and everything it represents. In the ensuing weeks, you keep everything from the glovebox to the wheel wells in perfect, gleaming, and tidy condition. Your friends and family learn not to even think of bringing fast food or cups of coffee inside. You might even find yourself taking up two parking spaces for fear of dings. But that special gleam doesn't last forever. One day you find yourself driving while chomping a burger, spilling ketchup on the upholstery. The car wash employees no longer know you by name and level and detail. Eventually, you notice a scratch and shrug it off. Your car has become familiar, just a way to get around.

Why does your new car stop bringing you the joy it once did? The answer may lie in what is known as the emotional baseline or happiness set point. Many psychologists suspect that we humans are born with a set level of happiness. This set point is different for each individual, but stays more or less the same for each person throughout his or her life. Though outside factors like acquiring the new Prius we've longed for might nudge our level of happiness up or down temporarily, we inevitably return to the point from which we started. This is called the hedonic treadmill, a nod to that exercise machine on which we seem to move, but actually end up going nowhere.

In a famous study, psychologists tracked two groups of people: those who had just won the lottery and those who had just been paralyzed in an accident. At first, the group of lottery winners were quite happy, while the paraplegics were understandably depressed or angry over their situation. Just what you’d expect. Over time, though, the emotional high of the first group began to dull as they became used to their new lifestyle. In contrast, the accident victims had also become used to their new circumstances, and treated paralysis as the new normal rather than a reason to be sad. Both groups had returned to their emotional baseline. Even the extremes of human experience didn’t knock them off their baseline for that long.

Just as a new car loses value the second you drive it off the lot, that initial burst of joy you get from buying it begins to diminish immediately. You slowly settle back to your emotional baseline, toward the level of happiness that you seem to be set for. The evidence shows that our level of happiness may be set genetically, so even many of the huge occurrences that change our lives—getting married, having a child, becoming wealthy—will not change our basic emotional set point. It's perfectly natural to get a boost in your happiness level from both these big life changes and new material possessions, just as it's natural for that same boost to wear off over time.

Humans are great adapters; it's part of why we have become so evolutionarily successful. It's also why we find ourselves on the hedonic treadmill. We readily adapt to our novel circumstances—whether it's a shiny new sports car or an exciting new job—and start to crave more. That excitement, without fail, gives way to habituation.

However, the hedonic treadmill doesn't affect everything equally. Some of the newest research into happiness demonstrates that our priorities can shape our general level of satisfaction with life. Perhaps unsurprisingly, people who prioritize helping others or spending time with family over more materialistic goals seem to raise their baseline level of happiness slowly but surely. So while the shine will come off a new car, it seems that the joy of helping others might keep you glowing for quite a bit longer.

Does this mean that you should never buy new things, or that you should feel stupid or naïve for enjoying the thrill of having just acquired something exciting? Of course not. Indulging your appetites for more and newer things is just part of being human. Savor the feeling of taking your new wheels out for a spin, and inhale deeply that new car smell. Just be prepared for the slow race back down to familiarity that comes standard. Your trajectory is set. Enjoy the ride.

The hedonic treadmill is very controversial. What do you think about the pursuit of happiness? How long after buying something do you become tired of it?