Why are some people so irresponsible with money?

About 4 months ago, my friend bought a new build 1 bed flat in London with an interest only mortgage.

She earns around £40k, and the flat must've cost around £250k with just a 5% deposit (so borrowing over 6 times her salary...)

As far as I know, she doesn't get big city bonuses, so I have no idea how she intends to pay off the equity.

And yet she runs a car, has just spent £800 on a ski holiday next February, went to Greece for a week in the summer, also went to Dubai (boyfriend paid, but she spent ALOT of money), splashed out god knows how much on furniture, and every weekend goes out drinking in expensive bars, eating out in posh restaurants, and then clubbing in "celebrity" style venues!!

Hmm, I have a hunch this could end in disaster, especially if prices of new build mass produced flats drop.