Well, I believe that most RELATIONSHIP dies not because the other partner cheats but basically because of they dont TRUST and UNDERSTAND their partner. We must be able to UNDERSTAND the likes and dislikes of our partner and TRUST their decisions for the big boss to keep flowing (LOVE)....

Sometimes people just grow apart

Yes they do but as a result of something...when it comes to that i advice counselling

In our case it was far too late, 50 years, we had an amicable split ,a shame but now we are both a lot happier

My thoughts are it is because people don't do due dilligence in picking a partner..
They don't ask hard questions, don't bother to make sure they have a lot in common, have the same ideology/ world view (for those that can't grasp this...say a tree-hugging pacisfist vegetarian and a NRA meat-loving conservative would have VASTLY different world views and ideology)

Seems most go for "she makes my boner tingle" and nothing else is important/ everything else (hard differnces) will just work out..
Then at the first sign of conflict..or when the "honeymoon" phase wears off....they realize they have nothing in common, nothing holding them together (shared goals/ vision), heck..don't even like each other.

My two long terms (8 and 12 years, am a widow now) were with nguys I carefully screened...that's why they lasted.

My ex, in thre 80's...that was one of those sad "boner tingle" things....and we had issues for years, because we were so different...and astime went on those differences became more glaring and caused problems.

So, I learned to avoid that type thing in the future, be more picky...and it served me well...

1. I will understand myself honestly and completely. I will remain true to myself and trust my own decisions because I have fully reviewed the subject at hand. There is nobody on this planet better qualified to be me than me.

2. I will not place my own expectations on another to fulfill. I must allow the other person to be themselves no matter how I might think they 'should' be. There is nobody on this planet better qualified to be them than them.

3. I will not attempt to seek from others, that which I should feel inside myself. The feelings I experience are inside me. They do not transfer to others no matter how hard I try to make them. They will never be able to feel my feelings as I do.

4. Other's expression of their feelings is dependent on how well I understand their display and how well they can express them. I will never be able to feel their feelings as they do.

5. I must remember that what I say cannot be un-said. What actions I make cannot be un-acted. I can try t repair the damage by saying or doing something to cover it up but if I use wisdom and forethought, no repairs will be needed.

6. I will not make more out of something than it is. I will not be ruled by my fears and insecurities because in knowing and understanding myself I have no fears and insecurities. I will understand that others may not be in control of themselves and may be ruled by fears and insecurities and I will work to comfort them.

7. I will not allow others to push my buttons. When they try, I will recognize it and give it no power over my calm. Likewise, I will not work to push other people's buttons. I seek no power over them, only friendship.

8. I will keep in mind to have a solid friendship requires that I be a solid friend myself. In the context of 'relationship' for this OP, these relationships all require some type of friendship.

9. In this Universe, change constantly happens. I understand that I change constantly. I understand that they change constantly. I also understand that when we are together, we both change constantly, both individually and as a pair. I will accept change, direct it and embrace it.

10. I know there are requirements that need to happen for a loving relationship. Some of the more important requirements are:
Respect for each other as they are. Respect in the unity.
Dedication to unity.
Commitment to stay focused on goal.
Honesty to self and the other.
Protection of unity.
Understanding of personality differences that can sometimes get in the way.
Harmony of essence in unity.
Accuracy of expression.
Prevention of manipulation.

Most important....Celebrate the joy of the unity.

If you let the little things get under your skin and build up till they explode, the solution is to not let the little things get under your skin.
Just let them go but if you can't, choose the words wisely and speak them kindly.

wow - 50 yrs is a long time - but so glad that you had an amicable split