I think endings are one of the main differences between books and real life. When writing a book, you get to decide, "This is the point where you know everything you need to know to understand these characters and this story."
But in real life, you never really reach that point. Sure, you may break up with a lover or end things with a friend. You might even never see them again. But years later they'll pop up on Facebook. Or you'll hear their favorite song, and you'll discover you never really listened to the lyrics before, and they cause you to see this person and your time with them in a different way. You'll realize your story wasn't really over.
I love this about book endings. It makes them so final. So concrete. So hopeful, that we can all, eventually, come to some place of true understanding.
It also makes them really hard to write. Because the best book endings hint at the real life conflict. You know what you need to know, but you also believe that these characters will go on, will continue to grow and change. And as a reader you are left with the bittersweet wish that you could be there to see that too. Even though it's over.