Your lover enters this relationship with some idea -- conscious or unconscious -- that they want to go about romance in a new way; they want to find someone who's going to be attracted to them for who they are, and not just for their surface-level appeal, like looks or career or the other usual factors of eligibility. This is a lofty ideal, to be sure, but as your connection deepens and matures and you both try to fit the relationship into your own idea of what a romantic relationship should be, you both slowly come to the realization that there's really not a whole lot of natural attraction between you. In your partner's determination to avoid yet another relationship based on superficial factors, they find that they've ended up going too far.
You might even feel as if you're basically just friends! Your emotional expectations may not be met in this relationship, and sparks don't fly between you naturally -- so you just have to make them fly, if this ends up being a relationship that you both want to hang onto. This problem isn't insurmountable; you just have to face down your romantic ideals and then give them up in the face of what you've got in front of you, in stark reality. Reality is better than idealism, anyway, in the end; reality delivers, whereas an ideal is always just that -- an ideal.