As a writer, we learn a lot about grammar and story development, making us good at editing other people's work. The transformation from writer to editor is relatively painless unless you have a problem telling people the truth about their work, but that's what they pay you for.
I know many writers turned editors in the industry and many of them prefer it over writing. It's certainly less stressful to critique someone else's work than it is to listen to someone critique yours. Editing goes far beyond simply making sure commas a used appropriately.
It also requires that you examine their characters, plot, paragraph mechanics, etc. You need to take what they have and polish it into something that is publishable. You need to take the same care that you would use polishing your own story. In fact, several editors continue writing their own stories on the side.
When the stories become popular or win contests, your reputation as an editor grows. Editors get clients just as much by referrals as anything else. When you have satisfied customers, you have people that will tell their friends.
You may think that editing may not be lucrative, but a good editor can make a decent living. It won't be easy and it may take several years to get your reputation high, but it can be worth it. I guess the moral of the story is if you want to be a professional book editor, don't start by quitting your day job and instead work up to it one project at a time.