Germany greatly benefits from the Euro since it makes their exports more competitive compared to if they had to go back to using the Mark. Profits higher and salaries lower. If it stops at Greece, fine - but Germany doesn’t want the entire EU to unravel.
Northern Europe has always felt duped by the Mediterranean countries that came on the Euro. Basically, they cooked the books to get on the Euro. I am sure they have the "don't let the door hit you in the a** on the way out".
Then countries like Belgium and the Netherlands look at the Scandinavian countries that are not on the Euro and see they are doing well. It would not look good if the capital country left the euro. I am not saying this will happen soon, but it is not good that Belgium regrets the Euro..