> take responsibility for their own happiness
On the list of things that are impossible exists an entry around trying to make other people happy. Literally nobody else can make you happy - only you can make yourself happy.
> They are individuals with a private friendship, they are not a unit or a completion or extension of each other
I am friends with my spouse, but we are also a unit in the sense that we are capable of acting in different cooperative ways to accomplish a single goal whether that's on the macro level of managing finances or a micro level of handling an emergency like a kitchen fire.
I would agree we don't "complete" each other in that we're both complete human beings without the other one - but we do extend each other and broaden each other.
> The breakups happen almost invariably...
The happiness ownership problem is a critical one, but it's not the sole root cause. A common related issue in relationships that I see is when one person wants to be infantilized by the other who ends up playing "the adult" - and that's not functional for anyone involved.
Another critical problem is the inability to meaningful communicate - I once had a pair of friends breakup, only for both of them to complain to me that the other one was never interested in having sex...
But it's a fools errand to narrow down the causes of relationship failure to a finite list.