I read a Tumblr post about this that worded the issue better, but I didn't 'like' it because I didn't really agree with it and now I have no way to find it again...
Anyway, just because I didn't like it doesn't mean I don't think it had a valid point...I can see how finishing a project and leaving those characters and the world behind can be kind of sad, like graduating from school...maybe you didn't have ten tons of fun 24/7 being there...but it was like a home; you got used to it. And now that time is over...
I usually feel that way in the middle of a story. ^^; When everything is at its most exciting, the main characters are all established and I'm still looking forward to writing the 'juicy' parts, I just think, "Man...creating this is just so much fun; I never want it to end..."
But honestly, by the time I'm done with the story, I'm very done. I've worked hard to make the ending make sense and close off all the character arcs, and I really need a break. It usually takes a while before I have the energy to even think about a sequel; the relief of finishing is too strong. ^^;