Part of being a writer is learning that no matter how wonderful you think a scene, a character, or even an entire novel is... that other people can and probably will think differently. When you invest so much of your life and heart into one work, you literally become unable to separate yourself from it in any useful fashion. I see truly talented writers get lost in a sea of obsession over a single piece, a single character. I've seen good novels go in the toilet because of a single scene that detracted from the meaning, but the writer was too invested in it to cut it.
Being a writer, a successful one anyway, means being able to step back and look at your work objectively.
Not only that, if you spend all your time and effort on one tale, your other tales get lost by the wayside. When and if you finally get published, your agent and your publisher aren't going to want you to stop there. They'll want you to keep writing, because that's what a career is: writing books. Plural. So if you've spent all your life writing one, you're not going to really KNOW how to write the next.
All writers need distance from their work in order to really be able to improve it. I spent five years away from Nightblade's Fury. That was probably longer than I should have gone, but my life was tumultuous. When I did finally pick it back up, I was able to read it with fresh eyes. If I couldn't see where something was going, I knew that my readers certainly wouldn't be able to. It gave me perspective, and I was able to whip out my red pen and brutalize it.
Here's what my novel looked like when I picked it up after five years and started reading:
Do you really think I would have been that brutal with it right after I was done with the first draft? I cut whole chapters... probably half the novel, lost to the red pen. But when the chaff was gone, I had wheat. And I had a product that I was actually not ashamed to show to others. It's not perfect, and still needs a lot of work, but I think that the end result will be great, and salable. But I won't stop there; in fact, even though I'm not done with this one, it's out of my hands, and I'm already working on the next project. An object at rest tends to stay at rest. Thanks to my completion of Nightblade's Fury, I've got motivation to really work on other things... I want a backlist of novels to submit. When Nightblade goes out to the agents? I'm going to keep writing. Because it may not get accepted, no matter how great I think it is. I don't want to put all my eggs in one basket.