Greece joined the Euro and this allowed them to borrow in the debt markets at a much lower interest rate than they should have been able to - people assumed that as they were lending Euros all of Europe was guaranteeing their debt.
Therefore they borrowed a lot, hid the state of their finances, collected very little tax, and the Government were "very generous" to "their friends" i.e. there was a lot of corruption. When the reality of the situation came to light, they needed a bailout.
This is where the situation get's complicated! There is an argument to be made that the bailout was designed to protect Europe rather than help the Greeks and that the conditions imposed are too harsh and have made the situation worse for the Greeks.
Greece should never have been allowed to Join the Euro when they did, their Government finances were already in a very bad place, and they had unsustainable spending levels, very low pension ages, high wages for government workers, but nobody paid taxes.
Unfortunately the problems with Greece were created by the politicians and the rich but it will be the general populous who suffer.