The two paragraphs preceding the money quote add valuable information:

Varoufakis is the newest finance minister in the Euro Group; Schäuble has served the longest. Varoufakis is a professor of economics, a man always good for a clever turn of phrase and a beaming smile. Schäuble is better known for being caustic and irritable. He is a lawyer by training and prefers practice to theory; he is matter-of-fact and deeply skeptical of those who seek to grab the spotlight. And he doesn't hold university professors in high regard.

Since Schäuble has gotten to know his new colleague from Athens, his appreciation for economy professors has dropped even further. He is suspicious of those who believe in their own theories and who think that the world is predictable. For Wolfgang Schäuble, societal behavior cannot be easily explained, not even by social scientists. That is why, he believes, negotiated rules -- and adherence to those rules -- is the best policy.

For Yanis Varoufakis, the euro is a defective currency. For Schäuble, it is his legacy.
(Money quote my bold)

Pacta sunt servanda is the only ground on which Schäuble can really stand. He dismisses everything else. This is sustainable for him only because, as the representative of Germany, his duty was to protect the integrity of the rebranded Deutchmark - like a good accountant doggedly protecting the integrity of his company's books and accounts - all else be damned. His attitude is as understandable as it is inappropriate to the present situation in the Eurogroup.  Change can only come with a change in leadership, or when, as last week, Merkel overrules Schäuble.

"It is not necessary to have hope in order to persevere."