That guy -- the perfect package, the one who's ambitious and emotionally articulate and gorgeous -- doesn't actually exist. And if he happens to embody all three of those qualities, I guarantee you that he has other "imperfections".
Because we're poorly educated about transitions in our culture, we mistake the fear for doubt and thus begins a scary domino effect of believing that we're in the wrong relationship. The message is: If you're doubting, you must be settling.
When engagement anxiety kicks in full force and causes people to nitpick their fiancé and put his or her every perceived flaw under a microscope, a downward spiral of focusing on what's missing usually begins.