It's a tough decision for a writer to make, one of the toughest. All your life you've fantasized about one of the big New York publishers buying your book and its subsequent astronomical launch into the stratosphere. But it hasn't happened yet in spite of your eating, sleeping, and researching the craft of writing for years.
While the non-responses (don't) come in, and the polite rejections clog your inbox (and your arteries, or so it may seem), the option of self-publishing hangs out there like a flag unfurling in the corner of your eye, waiting for you to take notice and give it some attention. It's Plan B and has been for a long time.