When I finish a book, I always expect there to be a sense of pride and accomplishment or sadness and emptiness at leaving the book and its characters behind, particularly at the end of the series. But almost every time, I feel nothing. Not like "numb" nothing, but like... just nothing. I mean, I love writing, I love the world and characters, otherwise I wouldn't spend so much time there, but when I'm done, I'm just... done.
For me, writing is the same as having to pee really bad. Like holding-it-for-the-last-hour-of-a-really-long-car-ride bad. There is an intense urgency and immediacy when I write, like I need to get it all out this second or the world might end.
And then there is something very satisfying in the act of the writing (much the same way there is in peeing after holding it for a really long time), but then when I'm done, I don't feel proud or sad. It's just something that I had to do, and I did it, and now I'm moving on.
That is best analogy I have ever come up with on what writing is like for me. It's like a biological function that I need to do in order to exist, and while I enjoy it most of the time when I'm done, when I'm done, I'm just done.