In the beginning of another great book by Seth Godin, so all I have are a few words about it. 

Basically, it's a book about knowing when to quit.  Yup!  QUIT.  Why do we need to know when to quit?  Because if you're spending all your time zeroed in on X, then you're not paying attention to Y...and Y is where you need to be focused. 

It's a different concept of the idea of quitting here.  There's more than one type of quitting.  There's I-have-zero-pereverance quitting...there's I'm-gonna-sit-on-my-couch-and-watch-tv-for-the-rest-of-my-life quitting...and then there's the type of quitting you do when you realize that what you're presently doing with your time and energy will only and always give you mediocre results and so you drop it and move on to something that you can be awesome at.

We need to know when to quit ventures that aren't going to give us the results we want.

BIG winners are people who have developed the awareness to know what to quit and what to pursue.

Steve Jobs quit.  He quit college so that he could pursue only that which interested him.  He called that choice: "one of the best decisions that I've made in my life."

Michael Chriton quit a medical career to become a writer and created ER and Jurassic Park.

Sometimes you need to quit what you're okay at so that you can succeed at what you're awesome at.

We don't like quitting because we associate it with failure.  Here's what J.K. Rowling has to say about failure:

Failure meant a stripping away of the inessential. I stopped pretending to myself that I was anything other than what I was, and began to direct all my energy to finishing the only work that mattered to me. Had I really succeeded at anything else, I might never have found the determination to succeed in the one area where I truly belonged. I was set free, because my greatest fear had been realized, and I was still alive, and I still had a daughter whom I adored, and I had an old typewriter, and a big idea. And so rock bottom became a solid foundation on which I rebuilt my life. – J. K. Rowling, Harvard commencement addres

Don't quit when it's hard.  Don't quit because it's hard.  Quit because you want to succeed at what matters to you.