People are constantly evolving and growing. What one might call their "ideal mate" at age 16 can be completely different at age 26, 36, or beyond. Our wisdom is shaped by our experiences in life.
Oftentimes two people aren't in the same mental and emotional space. A guy who wants to play the field or date several women is not going to suddenly become "marriage minded" because he met a woman who'd make a good wife. In fact odds are he would not see her as being "the one". Maybe years from now when he's ready to settle down he may wish he could find someone like her.
Timing is everything!