Not saying the former greek leadership is without blame, but as mentioned earlier, they never should have been allowed to join. Then based on overly-optimistic projections about 'growth', banks from other countries 'lent' them unreasonable (risky) amounts of money. Which of course (suprise) they couldn't pay back.
Rather than force the banks/lenders/speculators to take a loss (apparently that isn't an option anymore) the IMF steps in to the rescue with - you guessed it more loans. These loans however were used primarily to pay back the original loans (90%). The loans from the IMF comes with strings attached (austerity) which only makes the situation worse. When offered another loan/bailout you can understand why they might be inclined to say no thanks. Default and rebuild is their best option. Not going to be easy, but still best.
If you would like to read more about how thw whole IMF racket works, I suggest Naoimi Klein's - The Shock Doctrine.