Let's say you and I and a friend need some money.
We borrow from each other and swap the debt around and as long as the chain is good and we *trust* each other and pay back things run smoothly. Let's say I owe less but say I can't pay you 2 back. Then comes fear with you and the other guy saying to each other you are no good.
This sounds silly and nonsensical but happened in the USA in 2008. Psychology as no one trusted each other and said they were no good the assets instantly turned into expenses that no one could afford.
Yes, Greece is tiny. But what is to say spooked investors who lost tons of cash now look at Ireland and Italy next. Everyone does it and now Italy is no good because the other investors said so. It can't borrow to pay its bills, etc. Spain, then eventually the US.
Swapping debt assets seem stupid like the children's hot potato game where as long as you are not holding the potato when the bill is due you gain.