Everyone wants to find a mate that is attractive, affectionate, loving, genuine, trustworthy, honest... That's the ideal when it comes to looking for a long-term commitment with a lover. That's certainly what each of you is looking for, and with this aspect, you might start off thinking you've found it.
The attraction is certainly there, at least at first; but after a while, you might actually find that you've misread things. Your lover could start thinking you're more vain and superficial than genuinely attractive, and that flirtatious bond that once united you could begin to wane. It's hard to stay attracted to someone when you feel as if they're more interested in themselves and their own looks, their own lives, than they are in you; and unfortunately, that could be the case in this relationship for your partner. Even if you started out strong, a certain lack of trust gradually starts to erode your sense of togetherness. This relationship might even erode your lover's sense of self-confidence; they could start to worry that they're not attractive enough for you, because they just don't know why the attraction started out strong and then waned. If the attraction lessens, it might be a sign that you both need to put a little more effort into letting each other know that you're in it for the long haul -- and if you're not, then talk about that too. It might simply be time to move on.