The simple answer is that they never learned how to live peacefully with each other.  Today if you know how to walk, it’s because some day you learned how to walk.  Today if you can speak a particular language, it’s because some day you learned that language.  Same is true with cooking, driving, playing a musical instrument, etc.  Anything that you know today is just because some day you learned that.  So, why should it be any different for maintaining a peaceful relationship?  If you learn how to maintain a peaceful relationship, you will be able to maintain a peaceful relationship. That’s what I teach.