It's all about effectiveness. They intended the reader to get X from that line/paragraph/section/chapter. You, the reader, got Y. What would they need to do to get the effect they want?
The text might need a bit of rearranging. Too much of the game is being given away too early. Move that line, or take out that paragraph, and tension is increased. Sometimes it's about clarifying a phrase. Sometimes there's a stray word that's giving the wrong impression, or a place where an additional adjective would help understanding. An outside view is often good for the bigger picture - is a particular character necessary, are certain events too close together, is that the best place for a chapter end?
But above all, there is a sense of common purpose. We both want the writing to be the best it could be. I'm not 'the enemy', I only ask questions to clarify things. Does ABC work best? Would BAC be more interesting? What about CBA? Maybe ABC is best after all. It's such fun to play with another writer, letting the ideas bounce backwards and forwards. There aren't any right answers, it's not about getting another writer to do it 'my way', it's about firing up the creativity of the other author when perhaps they have come up against a brick wall or need to know how their work is coming across.
Luckily, the receptive student who wants to learn, who wants to listen, who wants to play is much more common than the negative student who sees the mildest comment as a personal attack. To all those who have allowed me to join your journey for a little while, thank you.