Also, don't underestimate the role of emotional connection in long-term physical attraction. When two people are able to connect and jive on an emotionally-intimate level, the human mind is capable of either overlooking or finding appeal in their beloved's imperfections. You might not have given them a second glance without knowing them, but after making a real connection and sharing yourselves together, suddenly that snaggly tooth they have makes their smile so charming, that extra pudge around their middle is the best pillow in the world, and the way that they snore in the middle of the night is practically a lullaby, because it lets you know that your love is beside you and safe.
When you're in love rather than simply in lust, you find that the things you once considered to be your partner's "flaws" are integral parts of the person you adore, and as such, you love those flaws as well.