I think it's a symptom of consumerism and materialism that we all want the latest and greatest. The car you bought a few years ago (barring any mechanical faults or such) is as good as it was back then, but advertisements and such telling us how good a new car is makes us want the newer, better one. I mean, you gotta have it, right? It will be much faster, safer and more spacious, or whatever.

Things telling us how much better it could be make us dissatisfied with our lives and our possessions.

Additionally, when everyone lives at a fast pace, a new car (or other possession) is a relatively easy way of fooling both others and yourself that things are changing for the better and your life is productive and worthwhile. New cars are indeed often like fashion, as we try to get the car's image to reflect back on to us, in some way.

Then again, there may be those who are less concerned with cars than myself, and maybe there are those who do own their cars until the wheels fall off, as Osna said. Maybe they are those who spend their money on other things, instead..