I think it's important to remember to celebrate things. A lot of people get so wrapped up in achieving things that it's hard to take a moment to enjoy the things that you've already achieved.
Not me. I freaking love celebrating stuff. Whether it's my birthday, my half-birthday, the fact that it's Friday, our anniversary (which it is today - happy 5 years, honey!), or just a particularly good hair day, I like to take the time to appreciate it. And eat dessert :)
So I think you should apply this theory to your writing. Yes, obviously celebrate selling your book to an editor or making the New York Times bestseller's list, but you don't need to wait for those monumental milestones to congratulate yourself on a job well done.
Some days, that's finishing a book. Some days, it's just getting 5 minutes worth of writing time that didn't suck. Some days it's thinking up a brilliant title for the next book that you haven't even plotted out yet. Some days it might be just thinking up a particularly witty tweet - or having a NY Times bestselling author tweet you back.
Writing can be solitary and it can be lonely and it can be HARD. So if you aren't finding the joy in just sitting down to write (which, some days you won't), then find the joy in anything at all. And celebrate it.
You're worth it! Because, gosh darn it, you're a writer. And the world needs more of those (and fewer reality TV show stars!) So that alone is worth celebrating (unless you are a reality TV star, in which case celebrate your 15 minutes of fame).
So go have a cookie on me! You deserve it!