Once you realize the person you are with does not possess the traits you want/need in a mate or you see that you have different visions for the relationship.
People don't change unless (they) are unhappy.
The goal is to find someone who shares your same values, wants the same things for the relationship as you do, naturally agrees with you on how to obtain those things, and last but not least have a mutual depth of love and desire for one another.
"Never love anyone who treats you like you're ordinary."
- Oscar Wilde
There are only two ways to experience joy and peace of mind in relationships. We either get what we want or we learn to be happy with what we have. Accept them as (is) or move on.
The choice is up to us!