I suppose it's human nature to treat "the new" better than the "tried and true" whether it be products we purchase or the people who come into our lives. We make more of an effort to "impress" or win over a "new" someone than we do to keep them.
Sometimes attraction fades because appearances change. Maybe the man has gone bald or the woman has gained 50lbs. Neither one of them probably would not have dated the other early on if they weren't attracted to people with those features/characteristics. However since they exchanged vows for better or worse along with finding other redeeming qualities they stay in the marriage and continue to love them. However if you were to have them describe their "ideal" mate's appearance it may be different from whom they are with.
Another factor that also can diminish attraction has to do with things outside of appearances. After having multiple disagreements/fights, seeing a difference in attitude or behavior, being treated differently/neglected, or having other mental insights about an individual this can cause us be less attracted to them overtime.
When we first meet someone we go through an "infatuation" phase where we believe they are "perfect". Real love doesn't begin until after we see their "authentic self".
“Love isn’t finding a perfect person. It’s seeing an imperfect person perfectly.” – Sam Keen