Recently another writer told me that she cried when she finished writing her book. I thought it was interesting because I often do that too.
Perhaps it's because the ending was sad.
Perhaps it's because we just finished what seems like an eternity of work and exhaustion takes over.
But, I think for me, it's like losing a friend. It's like a scene at the airport: you know you'll see your friends and family again but you cry anyway.
My characters are dear to me, sometimes I know them better and have spent more time with them (in my mind) than I have with my real friends. I know my characters are not real. I know that I wont walk into a shop someday and see them. When I finish a novel I feel a sense of sadness knowing a chapter (no pun intended) of my life is done.
I miss them. Often their life, their story continues in my head and out pops the sequel.
Maybe I'm alone in feeling this, but, I don't think so. In my world, the population is just a little larger.