I came up with a new idea recently. Ask any writer and they will tell you how exciting this is. The blossoming of a new idea is like seeing a guy for the first time and falling into instant infatuation. You don't know him from a bar of soap, but he could be the one. A new book idea is exactly the same. Your heart races, you get fluttering in your stomach, the world around you glows with possibility because this book could be like nothing else you've ever written.
So you sit down and start writing. And its just like a new relationship. Everything is fun and exciting and makes you happy. Things will be like this forever, how could they ever go wrong when you feel this way? The words flow, the characters are deep yet witty, the plot is genius. You burn through the first few chapters no problem.
Then one day you sit in front of the computer and it doesn't seem quite as easy as it did the day before. Just like when your relationship starts getting serious, the shine has worn off. You get annoyed with your characters because you suddenly realize your Alpha hero is actually a Beta and your heroine is swinging toward TSTL (too stupid to live). But that's okay! You can fix it because you have love, and that's all that really matters.
So you keep slogging on. You reach the middle and things start sagging. Worse, you realize its starting to get really hard, but you've still got a long way to go. Its an uphill battle and you start double-analyzing everything your characters say and do, looking for ways they're going to stab you in the back with their inability to lead the plot in any sensible or entertaining direction.
At last you get to the climax. You rise above the problems that have come before and can see clearly for the first time in a while. Maybe things aren't as bad as they looked on the ground. You remember that feeling you had when you first came up with the book idea and think maybe if you really commit and put your whole heart into it, things can be that way again, you can make it into what it was meant to be.
So you finish the book, like getting married, because those hopes and dreams still exist. And once the book is finished, things will be different, right? They'll be better? Because surely with a finished manuscript, the world will be a better place. And for a while it is. You bask in the gratification of having a completed book.
Except then you realize that you're now stuck with it forever and it needs to be edited. Torn apart and put back together, stuck under a microscope and a spotlight directed on its flaws. Like couples therapy, you don't want to do it, its going to hurt and make you face sides of yourself and your book you'd rather not see. But deep down you know its for the best, that when you come out on the other side, you'll both be in a better place. So, you take it one day at a time and hope you have more good days than bad. Some days, you and your book won't want to see each other, you won't even remember why you liked it in the first place. But most days, you'll tolerate it. And a lot of days you'll see the light side, the fun side. Occasionally something will remind you of that initial puppy-love you had for it. And in those moments, you'll remember why you go through this crazy process over and over, every time you get a new idea and start another book.