Why is it important for your SO to convert to Judaism?

A friend of mine is recently engaged. His fiancee is Jewish, and is adamant that he convert and give up his Christian traditions. This is not the first time I have heard of a relationship where one person is requiring the other to change their religion in order to get married.

It seems as if this request is prevalent in the Jewish faith. I’m just curious as to why this is such an important ritual for couples where one is not Jewish.

Has this happened to you? Did it improve your relationship, or cause issues? Does this occur more frequently in the Jewish faith?

and I am not asking HOW to convert, I am asking why it is of such importance