Write with the door closed, rewrite with the door open.
Your stuff starts out being just for you, in other words, but then it goes out.
Once you know what the story is and get it right — as right as you can, anyway — it belongs to anyone who wants to read it.
Or criticise it.
— Stephen King, On Writing
This advice comes from Stephen King's very readable memoir on the craft On Writing, which is, I think, at the heart of what it means to work as an editor.
You start out applying yourself to the task at hand with your own instincts, your own opinions and your own creative tastes. You have to, there is no other way.
You have to edit with the door closed. At first.
But then you must find a way to edit with the door open; to welcome changes, be open to disagreement with your way of doing things and to fundamentally make yourself servant to the director's vision, the client's requests or to whatever is 'best' for the project.
This is a difficult dance to perform.