You could put it on a number at a roulette wheel. If you won you'd get a huge payout.
Or you could put your money on a hand of black jack. If you won't you'd get much less money.
The reason is that on the roulette wheel your odds of winning are about 1 in 36. Meanwhile with a decent system your odds of winning at blackjack are about 48 percent.
In other words your reward is higher the higher the risk is. And you'd never complain to a casino that the payout at blackjack should be the same as the payout at roulette. You chose your level of reward when you picked your level of risk.
More importantly you don't complain when you lose money. The only reason there is the potential to make money is becasue there is the potential of losing it.
Lending money works the same way. When a bank or a country make a loan to a country they are paid an amount of interest that is is comparable with the amount of risk. They could choose to lend to say, America, where they are nearly guaranteed to get paid back but they wouldn't get paid much interest.
Or they could lend money to Greece and have the chance of being paid back much, much more than if they lent money to America, becauce the risk is much higher. When they choose to lend money to Greece or any troubled country they go in with the idea that they might not be paid back, but did so anyway for the potential reward.
In other words they acted exactly like you when you walk into a casino. And when you leave a casino with empty pockets you don't blame the casino, you blame yourself for not making the right bets.
Same goes for the Greek creditors. You can't take a risk because you want a reward and then bitch when your bet doesn't pay off.