I recently read an article on some human blog about the difference between the writers who finish manuscripts and the ones who don't--if people are predisposed to be novelists. You can check out that article here if you'd like. I, however, will be talking about why writers already have this trait and why they need more of it.
There are many factors of motivation. I'm going to outline them in terms of, of course, world domination.
To Achieve: I aspire (and will) to rule this planet.
For the Sake of Doing It: I love stepping on people in the process of my rise to power.
You Cannot Not Do It: World domination is as vital to me as eating and sleeping (literally, not figuratively).
Not Doing it Would Result in Something Unpleasant: Thor would have a kingdom, and I would not.
Certain definitions of obsession could fall into any of those categories. But I'm going to talk about the obsession writers have and need, which are very specific to the first two categories: achievement and enjoyment.
The Origin of Obsession
Where do we initially get the idea that we want to be writers? If you're like my alter ego, you may not actually remember your (probably lunatic) decision to begin writing. But the answer is still obvious: Books.
The Obsession Writers Already Have: Stories
It takes a lot of desire to sit down and say you're going to write a novel. It takes a lot of desire to say you're going to write anything at all. Because, as most of us have figured out by now (especially me with this world domination books. *rubs temples*), writing is not easy. It takes months of dedication and effort and heartache.
Of those three, heartache is the most important. Not necessarily the pain more than the willingness to put your heart on the line.
There are avid readers out there full of dedication and effort that are just as obsessed with stories as we are. Look at fandoms like HARRY POTTER, THE HUNGER GAMES, DOCTOR WHO, STARS WARS, etc. These are stories with huge amounts of fans who know every detail, who dress their toddlers as the characters for Halloween, who laugh and cry and angst over the books/show/movie all the time. So why aren't they writers?
As Obsession Grows...
So we have a lover of stories with plenty of dedication and effort. They have just made it over the first big checkpoint of developing and idea and sitting down to write it.
But they stop. Why?
1. They didn't have the ENJOYMENT.
Plain old didn't like writing. Maybe it's too much work. Maybe they're too critical of themselves. It doesn't matter. They've stopped. Dead end.
2. The didn't have a desire for ACHIEVEMENT.
So they like it, but that doesn't mean they want to do it again, all the time, or work to improve it. I, for instance, enjoy making and eating omelets. That doesn't mean I want to eat one for every meal of the day, every day of the week, and constantly read articles about and learn as much as I can about omelets. Though perhaps, at some point, I will make myself an omelet again.
The ones who stick around...
The ones who stick around are more likely to actually finish the novel. They enjoy writing and they have high hopes for it. They think their idea is good and they really like this whole "writing" thing.
They might not finish it either.
The Obsession Writers Need
We have the obsession with stories and motivation to write. What's left?
The last bits of obsession that take both of those already key traits to a whole new level. Maybe we can go ahead and call this passion. Just manic enough of a word to imply an obsession, but it also implies drive, ability, and work.
Passion for your story
My alter ego once wrote (actually, several times) first drafts of novels that she saw to the end but couldn't bring herself to go back and edit. Even more times, started novels that never progressed past chapters 1, 4, or 28. The idea lost its spark.
It's hard to know exactly if your love of your idea will fizzle out. In my experience (as I've had many attempts writing this world domination novel), you start out with interest and, if you develop a passion for it, your interest grows even more so as you write.
It is important to note, however, that even passions will slump. Writing is both work and a hobby. No one ever said you need to absolutely adore your novel 100% of the time you spend working on it. If you're drafting, sometimes pushing through that difficult part is all you need. If you're in revisions, taking a step back for a while and then reopening your manuscript may be all you need to be head over heels again.
Passion for writing
Querying that first manuscript does not always work out the first time. After all that work and emotional involvement, you can hit a dead end. It takes a lot of passion for writing to begin the process all over again. Not everyone has abs of steel, but all writers have hearts of steel.
Writing, just like any other pursuit out there, isn't for everyone. It would take someone very obsessed with stories to want to put in the effort to make one of their own. It would take someone very motivated to achieve and by their love of writing to see it through. And it takes passion to invest as much sweat, blood, and tears as a writer does in his/her manuscript.
Most of the writers I've met through blogs and Twitter already have passion. As a CP of my alter ego has said, entering those agent contests will give you heart palpitations, even on manuscript #2 and contest #bajillion. The feeling never gets old.
Congrats on being a wonderful, obsessed, motivated, passionate nutcase called a writer.