I can't speak for anyone else, of course, but when I am doing what I call "ice skating" writing, ie, when it is totally flowing along, I do get this feeling I could definitely describe as a "high". When I "finish" a fic I tend to get agitated because I'm not sure if it's really finished, or, more often, because I tend to get a lot of posting anxiety, like am I don't it right? (eg. right time of day, right places to cross post, right day of the week...) I do tend to get a second high when a fic hits whatever my "target" number of comments was. And that feeling is like how I feel when a plane that I am on hits the tarmac and starts taxiing safely. Which is like a relief without having had conscious worry.

I don't actually like that I have this target number of comments in my mind. But I cannot seem to stop myself from doing it.