With the formation of single common currency euro, countries like Spain, Italy, Portugal, Greece and Ireland benefitted. The main reason was the interest rates. Prior to euro being in existence these nations had to borrow money as a higher interest rate as there was different currencies and if the nations had to borrow money from other nations like Germany the interest rate would be high.
However post the formation of the euro currency they started borrowing money in Euro and thus were able to get money at lower interest rate at rate similar to Germany. Germany was one of the best performing nations in Euro zone and hence had to pay lower interest rates on its borrowing.
As the rates were low the nations as well as the citizens started borrowing heavily. Greece can be said to be the leaders in the borrowing amounts. The politicians and the policy makers in Greece started a huge social spending program;in this they subsidized large number of the public services. They went on such spree that they forgot the economics of the situation.