Perhaps Marius is feeling some of that same sense of privacy, of daring, because he says, rather more frankly than he ever has dared to say out loud, "No, they aren't nice."

--he immediately feels quite guilty. After all his grandfather has done for them? But the stubborn part of his mind reasserts himself: will he allow a sense of debt to his grandfather to make a liar of him? This is why one oughtn't contract debts, clearly.

"It was ours, and all the better for it. I think-- I think it shall be a shame, to have to go back to letting him arrange our parties." Because they will of course, even if he insists that it's all up to them.