You've completed your manuscript, revised it a couple of times, and now it's perfect. Right? As disappointing as it is to hear, you'll probably find something that could be better when you look at it again. Then, once those things are changed, you'll send it off to an editor or agent who may or may not accept it. If they reject it, you may get a request for revisions.
Revisions? Didn't you already send them a perfect manuscript? Uh, no. You see, even after your manuscript is accepted by a publisher, there will still be editing to make you "perfect" manuscript even better. Before you get angry that the editor doesn't see how perfect the story already is, take a look at the proposed changes and think about them objectively. Maybe that word would work better than the one you originally had. Yeah, on second thought a comma does need to go there. If you take a step back, distance yourself a little from your story, you'll find that the editor really does know what he is talking about.
So, how is it that this editor knows that changes need to be made? It's his job. Plus, he's not nearly as close to the story as you are. You've poured your heart and soul into your manuscript for months, even years. That's your baby, and it's hard to find fault with your baby. The editor understands this. The suggestions he makes are only to make the manuscript the best it can be so you'll sell a ton of copies and make readers happy. He's not trying to destroy your "voice" and make your story sound like a hundred others. The changes he suggests are not a personal attack on you or your writing. His goal is the same as yours: to make the story the best it can possibly be.
Once you and the editor agree that your manuscript is "perfect," it'll go onto production and hit the shelves of your local bookstore. You go in, buy a copy so others will see you with this great book and want a copy for themselves. Then you take it home and look at your baby now that it's all grown up and a member of the published world. You can't believe it. Right there on page 57, you find a sentence that makes you cringe. You can think of three better ways to say the same thing. How did that get through all of those edits?
Because that looked like the best sentence at the time. That's one of the hazards of writing and why your manuscript is never perfect. No matter how many times you revise, rewrite, and tweak your story, you will always find something you could change. Before you get paranoid that your story will never be good enough to get published, think about this. All of the books you see in the bookstore and the library were written by someone who thought it was perfect, went through the editing process, saw their baby published, and found half a dozen things in the book they wish they had done differently.
Once your baby is as perfect as you can get it, take a week or two off from it, work on something else, then go back to your baby. If you find something you want to change, go ahead. Just remember that you have to let go of your baby sometime and send it off into the world to sink or swim. If it sinks, keep it home for a while and evaluate why it's getting rejected. Get an objective set of eyes to look it over and help you figure out how to make it better. A critique partner is great for this. Then, once your manuscript is ready for it's next trip into the world, send it out with your blessing.
One of these days, someone will see your manuscript and fall in love with it. That's when you know your baby is growing up. When you see it on store shelves, you know it's grown up and ready to stand on it's own in the world, with a little promotional help from you, of course. But that's another post.