Jealousy, emotional manipulation, possessiveness and control could become major components of this friendship if you're not both careful. Although dominating you will never bring them real satisfaction, your friend will try it anyway, because their connection with you will feel so important to them. Similarly, your own need for true friendship could keep you locked into this pattern for quite some time, unless you can find a way to examine your feelings and needs and figure out why you've made the choices you have.
You two could be projecting your overwhelming needs for companionship onto each other, which will lead you into this controlling, emotionally complex situation. You'll both have to examine your emotional needs and your expectations of each other if you're going to maintain the positive parts of your friendship.