Amazing things can happen when you publish the book you've devoted so much of your time and life to, so being realistic about your expectations and defining what will make the book a success will save you from feeling let down and disappointed if your wildest dreams aren't met. Defining expectations helps you to enjoy and celebrate the accomplishment that your book truly is, rather than disparaging it because it didn't meet unrealistic goals. There's still room for the seemingly-impossible in your dreams, but defining the possible and how to achieve will make for a happier, more productive experience.
There's no reason, except the odds, that any or all of your dreams can't come true for you, but the total success at the level of Wild or Harry Potter happens to very, very few. Instead of putting all your efforts into winning the one-in-a-million author lottery why not also work towards being among the most successful of the other 999,999?
What do you want, really? Define your wildest expectations, don't be shy. And then start to look at how you can help these expectations become reality, rather than leaving it up to the book fates.
