Ugh, there are so many things!
#1 Is seeing people read the story when you post a new chapter, it still amazes me, looking at the stats and seeing all those people catching up to the newest chapter. I hope they enjoy it.
#2 Is getting any kind of comments. So far I've been trying to respond and like every comment because it makes me so happy!
Some others are;
When you feel like you wrote something that turned out better than you expected.
When someone favorites a chapter
When people vote in your polls
When you feel like you have ideas that are coming together
That giddy feeling of anticipation for when you get to some payoff, I'm so looking forward to people's reactions at some of the things I have planned!
Not taking it so seriously, it seems the more serious I take it the worse I feel. That doesn't mean I don't edit and care about presentability, more just, typing and letting my characters go where they want, it's like I'm discovering the story too!
I'm only like... 11 chapters in my first time so I might feel different later, who knows?