You should always be writing what you want to write. I’m a big believer that you should never write for something or someone other than yourself. Your should be writing because you want to, not someone else. That being said, I think that sometimes while we’re writing for ourselves, we forget what it really means to just write.
For me, this works like a miracle when I’m dealing with writer’s block. I take a story idea that I don’t have plotted out, and I just write. I don’t think ahead at all. I don’t worry about plot holes or explaining things. I write what I want to write because I know that no one else will ever look at this piece of writing. Now, that might not be true. I could decide that I love what I’m writing, but I still probably wouldn’t share it until I’ve fixed the plot holes and explained the character’s background.
When you’re just writing what you want to, it’s okay to stop at any time. This is a piece of writing that you shouldn’t feel obligated to continue after the first couple of paragraphs. You can, but you don’t have to. It’s like vomiting words onto the computer screen: no one expects anything to come of vomit. But, unlike vomit, writing for yourself is fun!
You can make a character completely crazy. You can add magical ninjas into a previously realistic piece. Want to throw in a wedding? Go ahead. Try it! It’s so much fun, and it also doubles as an idea generator. When you can do anything, you’re bound to come up with something for your current work in progress, or maybe an idea for something completely new.