Well marriage and relationship is all about understanding and knowledge.
Once there is no understanding there is bound to be problems and issues in that relationship.
So many courtship today fail to get to marriage basically due to lack of understanding between couples.
There are other things which can also cause courtship to fail, like when both couples start discovering new habits that they haven't seen earlier between them.
Lack of finances causing delay in marriage can as well lead to break down in courtship.