I only have one main reason why a relationship does not work out. (Selecting the wrong mate!)
Each of us gets to (choose) our own friends, lovers, and spouse. Our lives are for the most part the result of the decisions and choices we have made.
Very few choices rise to the level of importance as selecting one's life partner, and yet many of us never take the time to do the necessary introspective thinking needed to determine what it is we want and need from a mate or potential spouse.
If you are with the "right person" then you share the same values, have the same goals for the relationship, have (mutual feelings) for one another, care whether or not each other is happy, sexually compatible, and meet each other's expectations.
The reasons why people select the wrong mate are numerous. Sometimes they just want to be (with) someone/anyone. They haven't determined who they are and therefore do not know what it is they want and need in a mate. They thought that given time they'd be able to "change" him or her into the person they (really) wanted. They became "emotionally invested" before their mate revealed their "authentic self". In other instances they've created their "shopping list" for a mate but threw it out the window becasuse they felt he or she was soooooo HOT!
Selecting the (wrong) mate for one's self is and always has been the prime reason why a relationship fails. One man's opinion! :-)