World is globalized now, there aren't anymore crisis just for one country.
Recession in one country will impact all the rest of the world, especially when the big guys are involved, like US or the European union.
Your housing bubble in 2007 fucked us up all around the world, the euro crashing will do the same, probably even worse.
All the economies are intertwined and when something happens you get a domino effect, I'm not an economist and this is long to explain, but basically:
1 - Markets are global, companies are multinationals, and trusts fells in all the stocks generating even more trust issues.
2 - If a country is in crisis will import less goods and services, that implies other countries won't be able to sell all their production.
3 - All the countries own titles and bonds of other countries, if those countries default, what you have is just a bunch of papers.
4 - If the Euro crumbles isn't going to be only an economical crisis but a political crisis too.
