Why does Greece affect the euro?

[Angelo]

I consider myself a somewhat knowledgeable amateur in financial markets, but I can't for the life of me understand why the whole situation with Greece is such a catastrophe. Why do markets react to Greece defaulting on its loans like the end of the world, and why does the EU feel the need to bail them out? Everyone just says if they let Greece default without leaving the EU, then it will be a disaster for the euro. But why? Why does Greece defaulting mean the value of the euro goes down (significantly)? I can understand a little blip, but does the rest of the world suddenly not want euros because everything in Greece gets cheap? Is Greece really importing so much that its inability to continue to import will vastly affect the economies of other EU countries? I just can't imagine that is the case. Anyone else here have a clear picture on this?