If you're not stubborn, you'll give up on experiments too soon. And if you're not flexible, you'll pound your head against the wall and you won't see a different solution to a problem you're trying to solve.
In the end, I've found that it doesn't really matter who you marry. If you like them at the beginning, you probably won't like them at the end. And if you start off hating them, there's always the chance you'll end up thinking they're all right.