Like Just_A_Guy_You_Know, I am my own intended audience. Or at least that's how it started. It's been over a decade since I started typing up my longtime fantasies, often based on missed (or botched) opportunities when I was young, but only three years that I've been publishing them. Much of my initial motivation has simply been to get my ideas committed to paper, or at least to pixels. Maybe as I age I just want to get them documented before the memories recede too much further!
To be more specific about my internal gauge of quality, I'm not always confident when I click "Submit" that it's a great story. But the more times I've reread and re-edited the story, sometimes dozens of times. the more confident I am. I spent an inordinate amount of time reviewing and fine-tuning my Noir entry, "Dick Job." My initial submission was actually sent back due to a number of errors I hadn't caught on my own, after which I installed Grammarly to check my work and went through several more edit cycles. I ended up being highly confident that my final submission was a great story, and knew it was among my best work. I felt validated in that by the story's top-ten ranking, the only one I've had.
Despite pretending I'm doing this for myself, I have to admit I enjoy positive feedback when I get it. As others have said, Likes and Favorites are nice, but Comments are king. I do really enjoy hearing that a story of mine resonated with someone as a shared unfulfilled fantasy, or brought back fond memories of an actual encounter. Since most of my stories focus on male bisexuality, it's not surprising that many of these comments are from other men ... and guys being guys, quite a few have even stated frankly that my story brought them to orgasm. As banal as that may sound to some here, I take it as high praise. A number of my stories have also resulted in collecting new Friends and/or exchanging fun messages with them, and it's hard not to enjoy feedback like that.