I am absolutely the worst car owner to walk this planet.
I will desire a car; do everything to get it; then see that the grass is greener; jump and get the new car; rinse and repeat.
Saying that, I do not regret any vehicle that I buy - but I do regret losing the ones that I sell on (if it were my choice, i'd have kept every car, but i'd need a big driveway then...).
With most vehicles, it may be the aesthetics, figures, performance, brand, price or public's perception of your car that makes you want it. Sadly each of those are very limited:
Aesthetics - you cannot see it when you drive it.
Figures (MPG etc) - there will always be better and more economical.
Performance - this is circumstantial as even a hatchback can give you a surprise, all dependent on the gearing, conditions, driver and the risks they are willing to take.
Brand - once again, this is a materialistic viewpoint. BMW are a great premium car, but when you've owned a few, you come to accept and expect it. It becomes common to you.
Price - there will always be cheaper vehicles offering more bang for buck, especially as your car and tech ages, and newer models come available.
Public's perception - it is entirely pointless to base a purchase on what people think is cool or not. It could make it more criminally desirable, but also shows that your appreciation of the vehicle is fragile. It is your money, so get what you want, not what your neighbour wants or the kids down the street etc.
All in all though, I do get bored of the vehicle. And I use this term basically under the guise that Fox says it - I am used to the vehicle, it doesn't surprise me or impress me. It simply just delivers.