There can be several reasons why but I list some of the most common reasons and give you some solutions of you are willing to put in the work.

We start to disrespect one another and take each other for granted and it start out with small stuff and little digs and it gets worse over the years.
We let little fights and things that bother us build into resentment which festers into bitterness and hate.
But most importantly, we stop communicating and being considerate of one another. Think back to when you fell in love with one another how well you would communicate with one another AND how considerate you each were of the other. My suggestion is for one or both to watch the movie "Fireproof" disregard that its a religious movie please; you will take a lot from this movie, then go out and purchase the Love Dare book or Ebook and put it to practice for 40 days. I assure you, it will bring you back to the day you fell in love and wipe away all the petty resentment that has piled on over the years from one petty unresolved argument to the next from not communicating and the best part is that as you put it to practice your spouse will also grow in the 40 days without realizing what is happening just by you implementing these minute changes.
OF NOTE********Here is a Potential reason for Both MEN AND WOMEN is HORMONAL Changes AFTER 40. The Pharmaceutical Companies make too large a profit by selling medications like Viagra and Cialis and for Women they Profit off of , Synthetic Hormone Replacement Therapy(During Change of Life Cycles), or worse misdiagnosing/ improperly treating your Thyroid, Endometriosis, PCOS, etc... Good News is there are solutions for both Men and Women. I am not talking about FAD solutions either. Insurance does NOT cover the cost because the Pharmaceutical Industry can't make a profit if you actually feel better; I speak from personal experience and form being in the medical field for years.
If you are interested in learning more about BIOTE HRT therapy email me at peaks.inside0h@icloud.com with any questions you may have.

@dipen-chauhan

Dipen Chauhan