I've been dealing with this exact experience the last couple of weeks, after being asked to take ownership of a huge codebase. From what I've noticed though, saying "I don't understand" usually triggers a more detailed explanation of the existing design, leaving out the "why" part unanswered, and ending with me still completely baffled. I've had to resort more often to "Can't we just..." type questions, and while that elicits a more useful answer that makes everything click into place, I'm afraid of looking like an asshole who thinks he knows the answer to everything. Isn't there some compromise between these two styles?