A large source of currency fluctuations are trade imbalances. If a country exports more than it imports, its currency appreciates. If it imports more than it exports, it depreciates. For example, if USA imports from the UK are bigger than it's exports to the UK, then USA will be buying GBP with USD (on average), so the value of GBP will go up relative to USD. This would in turn make UK products more expensive to Americans, so they would buy them less, correcting the trade imbalance.
If Greece, Italy, Spain etc. still had their own currencies, they could simply depreciate them (actually, it would happen naturally, as a consequence of trade imbalance) and so reduce their debt in real terms (similar to what has happened to Iceland). If Greece's currency would depreciate, imports would be more expensive for Greeks, so they would import (spend) less, while Greek exports would be cheaper worldwide, so they would export (earn) more.
Conversely, Germany's currency would appreciate, again because of trade imbalances (they export more than they import).
Because of EURO, this hasn't happened, and won't happen - the trade balances are averaged, making Germany's currency (EUR) less valuable than it should be, and Greece's currency (again EUR) more valuable than it should be. This benefits Germany, because it can sell its cars cheaper, but isn't helping Greece, because Greek olive oil and tourism experiences are more expensive than they should be.
The short-term solution is for Germany to admit this advantage and help the under-performing countries (by giving them money, not just loans, and correcting the trade imbalances this way). In the long term, a single currency should lead to a single fiscal government (like in the US).