In the UK, borrowing has become a form of saving rather than the opposite of saving. People take out a debt and are then contractually obliged to put money aside to service and repay it. Legal contracts are used to promote the setting-aside of income because the social and commercial culture work against it. The two main motives for household borrowing are to supplement income at times in life when this is inadequate; and to accumulate assets (mainly houses) which provide a 'buffer stock' of equity if income drops below consumption, in retirement or before.
This is a very nice description, and perhaps describes the underlying cause well. On the other hand, it didn't feel like that very rational description, more a like a blinged-up, celebrity-wannebe madness.
I've also seen descriptions in terms of the distribution of economic growth, especially in the US. The mean or median inflation-adjusted wage has hardly budged for 30 years (in fact they seem to have fallen from 1972 to 2007). So, whilst those at the top have prospered, those at the bottom have not, and borrowing has been a way of making up for that. Given that the source of a lot of the borrowing has been the newly emerging low-wage economies, not only has the average UK/US worker lost their traditional jobs to the East, they have to borrow from those who now have those jobs in order to maintain a place in their own economies.
I suppose that these two descriptions aren't mutually exclusive, and indeed have fed off each other. They don't say much for the UK/US way of running an economy, though,