Why Does it Get Harder the Closer You are to Finishing?

For every writer who has ever written "The End" on a piece of work there is no greater thrill or sense of satisfaction during the whole process. I've missed that thrill for a great long while now. For a variety of reasons (my first book released last year, I submitted my option book last fall, I've had two WIP's that have stopped and started for a variety of reasons, my mom got cancer, we're in the middle of a major house reno, you know ... Life?) Anyway, I'm now approaching the last fifty pages of the book and I'm so close I can taste the end. So why the heck is it so hard to get there?

I'm thinking, for me, it's a couple of things. For this WIP I have a vague idea of the ending but it keeps shifting as my characters do unexpected things. That's okay, I can handle shifts but it means I need to add in another scene to get the right resolution. I can't just leave things hanging. I hate endings that don't feel complete. I'm okay with some open endedness but I need to close up some loops on a few characters and the action just hasn't stopped! I've had the first climax of one storyline and now I'm approaching the second climax. After that things get resolved fairly quickly. But approaching that climax? That's climbing the mountain, that's stretching the calf muscles a bit more than rolling down the hill toward the end. And that's always why it's harder for me to get to that finish line. There's the climb just before it.

How about you? Is reaching the end the easiest part of the book for you? Or is it a bit of a struggle?