I echo a lot of the previous sentiments regarding money, but just as important to me is the hassle of replacing my car. I'm a field sales engineer and carry a lot of equipment. It takes me quite a while to get my car set up the way I want it. Audio, XM antenna. hardwired radar detector, setting up the trunk, extra 12V outlets (both key off and constant hot,) certain upgrades, etc. Takes me months of spare time to get things just right. I only buy used these days, so I perform a substantial amount of preventative maintenance as soon as I get the car.
Also, I invest in things like a spare set of wheels with winter tires, factory shop manuals, maintenance supplies and special tools. I'm thrifty, so it takes me a while to accumulate this stuff while waiting for good deals.
There's really nothing on the market in my price range that is so fantastic that I'm eager to go through all that again. Plus, all the time spent means I really get to know the car and all the tribal knowledge of the model.