Maybe it takes a female science fiction writer to get it right, but the most compelling, resonating description of what it is like to pair with another person for life is at the end of Chapter 6 of Ursula K. Le Guin’s “The Disposessed”.
In it, Takver tells Shevek, “I need the bond. The real one. Body and mind and all the years of life. Nothing else. Nothing less.”
The rest of that chapter is achingly good, matter of fact and true.