I was asked the other day, "Why do you think mothers- and daughters-in-law don't get along? What is the core problem?" 

Well, it's not a core problem so much as a reality. There's a woman in her 40s or 50s who has given birth to, taken care of, been responsible for, and loved her son. She has been the woman in his life for decades, when in comes a sub-princess half her age and not very mature (because nobody is at that age) ready to dethrone the queen. The princess feels threatened by the queen because the queen has been the matriarch. By the same token, the matriarch is looking down at a whippersnapper who doesn't have enough experience on the face of the Earth to deal on an equal level. 

The mother- and daughter-in-law can never be equal, nor should they strive to be. They can be warm and loving, but it's always going to be a hierarchy. That is the foundation of why there are problems and difficulties. It's not because there is an inherent problem or that somebody is bad, it's that the situation is bizarre. 

Why is it not the same for men and their sons? Because the father/son relationship doesn't change. Why is it not the same for men and their daughters? Because they didn't go buy tampons together. Furthermore, the son-in-law is not dethroning the king - the king is still the king to everybody.