When someone 'settles', what that means is that the things you like about someone marginally exceeds those things you do not like. Over time, those things you do not like become bigger, and those things you do like become expectations.
Gratitude drops: dissatisfaction increases.
This is why men need to make sure that the woman they attach themselves to, from the start are nuts about them.... because over time attraction and interest will drop over time. If you fall for a woman that is just 'settling' for you, you are in for emotional pain. You do this by setting aside your ego, and evaluate a woman on her actions. If you never find this woman... don't sweat it... you are better off alone than chained to a chick that blames you because she settled.