Sagittarius natives yearn for an unfettered life, so they're free to explore the world and their own nature. In love, this doesn't always translate to a traditional dating life (aka, a stable, lasting union).
After all, you may have no idea where you're going to be next week - in town or on the other side of the world? You like to keep your options open in terms of your schedule, and in your relationships with others.
It's not that you're afraid of commitment. You just want to make sure you can hold up your end of the bargain if you're going to do it. If you meet someone special, be willing to let go and let them steer your wheel from time to time.