Many of us have experienced something at one point or another where we just don't want something to end, a TV show or a book or a movie...
Then again, there is the complete opposite, where you like (or maybe liked) something, but now it just needs to end.
It was all swell and great... at first. Now, the characters have changed so much since the beginning, and you keep introducing new characters... and what's up with that one? Yeah, he used to be smart and now he is just helping the enemy?
This is actually one of my fears as a writer. That I'll write a series or a trilogy and not know when is enough and when to end it. That is one of the reasons why I like to write stand alones... because sometimes for its own good, it needs to end.