Invested, Skeptical, Scrutinizing
The characters in the Republic are one skeptical bunch. Constantly challenging what others claim, they won't rest until they've completely understood a problem from every angle. If you think reading the Republic is exhausting, imagine being one of the characters in it. They're totally invested in the discussion, and they're not taking anything on faith.
You'd need that kind of investment to keep up the high level of scrutiny every topic in this book receives, considering the whole thing is upwards of 300 pages long. When's the last time you had a philosophical discussion that could have filled 300 pages?