Even if you aren't promiscuous in real life, you often are in your mind: You're constantly looking around at all the hot and intriguing people around you, and wondering whether they're better than the one you're with. And you suspect that they are; you've got a real grass-is-greener perspective when it comes to romance. You tend to get bored easily in relationships, and anytime things start approaching anything close to routine or predictable, you bail.
You just feel restless and itchy, and you have a little worry at the back of your mind that grows bigger when things start to feel too stable -- Is this all there is? Am I stuck with this one person, for the rest of my life? You might think that a relationship lasting into the foreseeable future would drive you crazy, and maybe it would; you certainly seem to have developed an allergy to stability in love. But if you want true intimacy in your life, you're going to have to figure out a way to make the long term work for you. You may tend to fall for aloof, unstable types, because at least they keep you guessing; try for someone more stable, but who is unique and independent, and can give you plenty of space.