"Hey, you opposed my article's FAC! What gives?"
This doesn't mean the article is dreadful, needs a ground-up rewrite, or is even less than excellent. An oppose vote is many reviewers' default position if they have two dozen or just two issues with an article.
"So wouldn't it be more genial to just post a list of suggestions?"
Where concerns and comments are posted, but no oppose or support votes are cast, it's difficult for the FAC director or delegates to determine whether an article should be promoted, left on the FAC backlog, or closed as unsuccessful. And it doesn't help you figure out which areas need the most urgent attention. It may even be appropriate for you to withdraw the FAC to give you time to work on any issues at your leisure. Without unambiguous feedback, you can't possibly know.
"So the FAC stays open longer; why is that a problem?"
FAC is under strain due to a lack of reviewers. It's not uncommon to see 50–60 FACs awaiting closure; keeping a FAC on the backlog when it could be closed dilutes the reviewer pool and increases the workload of the FAC delegates, who can spend up to 12 hours scrutinising the page for potential closures—only to find nothing suitable. When a FAC has no oppose votes, yet several concerns are listed, the delegates have to decide whether to leave the FAC on the backlog or promote the article believing the issues are minor and will be resolved. They can't read reviewers' minds to decide whether the concerns are sufficient to hold up the nomination, and when they do make a judgement call they inevitably have to spend even more time justifying it. Potential reviewers may also become demoralised upon seeing the backlog; the number of reviews drops and the problem worsens. In short, it helps everyone—including you—when reviewers are forthright enough to say they don't think an article is ready.
"What if I disagree with your oppose?"
There should be no obligation to do anything to the article to get a support vote. If you think an oppose is unfounded, then say so! Good reviewers will always be willing to reconsider when presented with a good editorial reason. FAC should be collaborative, not combative.
And remember, a lengthy oppose can easily turn into a support after only a couple of hours. Good luck!