Japan can also print its own money, which gives it the ability (at least in theory) to wipe out all public debt with the stroke of a pen. There are consequences to that action, but when your debt is counted in your own currency, you can largely ignore the public debt, as long as inflation is kept in check. Most non-eurozone countries (including the US) do the same thing. In fact, inflation has long been used by most nations to decrease the impact of public debt, as the fixed-dollar debt can be reduced to a smaller %age of the GDP. It's how Great Britian and the US 'paid' for World War II, for example. As another example, recall the talk about a "trillion dollar platinum coin" during the most recent US government shutdown, which would have paid off a trillion dollars of debt as well as instantly and drastically inflated the dollar (among other things).
Greece doesn't have a national currency. Their debt is in Euros, and must be repaid in Euros. Greece can't unilaterally inflate the Euro to reduce their debt load. The situation is closer to the economy of California, which is also heavily in debt. California doesn't get to print its own money, and it doesn't have the option of creating inflation to reduce its debt load.
Greece and California have two choices: Make their payments, and hope inflation happens on its own, or default and accept they won't be able to borrow money for a substantial amount of time. Growing their economies makes both options more palatable, but doesn't solve the problem by itself.