Although it naturally often seems to be a matter of a mix of factors, there is usually an overarching cause for a first long-term relationship or marriage not lasting.
The brains of young people are, partly due to a lack of experience, still very flexible and adapt easily to another person.
As soon as they fall in love with someone, they come up with all sorts of reasons why that particular person would be the right partner for them.
They feel attracted to that person by their perceived similarities, which make them find him/her very likeable.
But they are also attracted to that person by their differences, which they see as complementary qualities, and which they admire and respect in the other.
Falling in love is a wonderful feeling and therefore a strong motive.
Thus a chance encounter with one of the more than eight billion people on Earth can convince them that in this very person they have found that one ‘true love’.
And subsequently, that person often remains their true love for years, even for an entire marriage, through the forces of habit and memories.
Until their brains have matured more and they begin to realize that there might be other and better fish in the sea after all.
Often the situation is already complicated by then, and there are children.
After all, nature ensures that, driven by feelings of love and lust, we switch off our minds just long enough to pass on our genes to future generations.