“Lilmod, if a wife wants a divorce, and the husband hasn’t done anything terriby wrong, would there ever be a valid reason for the husband to say no?”
You have to ask a Rav (a real one -not an online one). Every situation is different. As a general rule, if one person really wants to get divorced, I think it would be the right thing to do. And beneficial to everyone in the long run. As a certain Rebbetzin I knew used to say, “better 4 happy people than 2 unhappy people.”.
But again, you should be speaking to Rabbanim, and not to online posters. There are too many people here answering your questions, people whom you have no idea who they are, how intelligent they are and whether or not they have any idea what they are talking about. It is too tempting to choose to listen to those whose advice you like whether or not it is the right thing to do.