Spain actually had better fiscal discipline than Germany during the boom, with lower government deficits and lower public debt, so "discpline/indiscipline" is definitely not the whole story.
The main difference I can see is just that the German economy is much stronger than the Spanish economy, not any difference in fiscal discipline. Germany can afford to carry bigger deficits and bigger debts because it has a comparatively strong economy; same reason the U.S. can maintain much higher deficits than Spain without borrowing costs increasing.
What matters in terms of financial responsibility is not public debt alone, or private debt, but both.
For the last 40 years if you built property in Spain people from Germany/UK would buy it.
How did Irish banks expect to sell more new houses than they had people?