Even if you're genuinely interested in this relationship at first, your interest could fade fast as you get closer with this person, because you'll come to realize that their vision of your connection is rather different from yours. While you want to have fun, they want to create the romance of the ages -- just as they did in a previous lifetime. They worked just as hard then as they will now to create a lasting bond with you, but now, as then, it won't work out; at least, not in the way that they want.
Any independence on your part will be perceived as unreliability, and will call up those old feelings in your partner of dissatisfaction, even heartbreak. Why can't they ever seem to pin you down? The answer, of course, is that no one can pin anyone down. You're either in the relationship because you want to be there, or you're looking around for some other lover or experience. This will be difficult for your partner to accept, especially if they're more interested in you than you are in them, which is likely. If you do have to let them know you're not interested in anything more than a casual affair, let them down gently, and avoid leading them on.