After hearing a psychologist on the radio talking about mid-life crises and even quarter-life crises, I developed a theory about the whole phenomenon. Of course, given that I'm not a psychologist myself, this should all be taken with a very large grain of salt.
My theory goes something like this: some people define their identities by things in their lives, most commonly their childhood environment, their job or their children. When these things go away (finishing school, children leaving home, retirement) we suddenly don't know who we are any more. So we thrash about looking for something else to define us, like a new, impractical car.
Mokalus of Borg
PS - I realise this would make mid-life crises more like identity crises.
PPS - That's assuming I know what I'm talking about when I say that.