Joseph,
The concept of “mazal” as it appears in Shas.
In this case in particular, people presume that because there is a person that you are supposed to marry as pre-ordained by the bas kol before you are born, your marriage to that person must be happy and loving. This is a mistake. A person can have mazal that he should be poor, mazal that he should be stupid, mazal that he should be sick. His bechira in life is then dealing with those unfortunate situations (or in some cases overcoming his mazal). So too with his “bashert.” Just because she is pre-ordained for him doesn’t mean that the marriage will be easy, pleasant or loving. It could be that his lot in life is to deal with a difficult marriage.