There is, however, deep structural or cultural issues that causes Greece to have such a massive tax gap: http://www.imf.org/external/pubs/ft/scr/2013/cr13155.pdf.
People may well be working hard, but a significant enough number of people are not contributing enough to the state, yet are demanding (by voting) ever more from it, and leaders seemed to have always been relaxed about it (I can't find the source again, but I remember reading that there is even a significant tax cap in the civil service itself).