Separately, you might be the most placid, easygoing people in the world, but when you're together, your aggressive sides will come out in a big way. You'll both constantly try to prove you're right, provoking needless conflict. In the middle of an argument, you may know exactly how silly your behavior is, or how ridiculous the argument itself is, but you'll be hard-pressed to pull yourselves back under control.
Something will just make you want to compete against each other -- force against force -- for domination. You'll find it almost impossible to set your egos aside, even when it comes down to what's best for you, your partner or the relationship itself. What does this mean on a day-to-day basis? Lots of arguments and hurt feelings. Lots of anger and aggressive, even hostile behavior. A hard time creating a lasting sense of harmony, togetherness, trust or intimacy. And while you may mistake this abundance of passion for the good kind -- the kind that makes it so fun to kiss and make up after a fight -- sooner or later, you might decide it's just not worth the heartache to stay in this relationship. There would have to be a lot of sweetness between you to counteract this level of aggression and hostility.