No, no, no, you misunderstand. It's not "Misery loves Company." It's much more simple than that:
People. Love. TENSION!
Think about it. Nearly every romcom goes down the shitter after the main couple gets together, or it changes focus to different things. Why? Because people like stories that have TENSION, and a solved issue just won't draw people to it the same way unresolved issues do!
Why did Gotobun get popular, despite the marriage being confirmed from the first chapter? Because TENSION AS TO WHO IT WOULD BE drove the people crazy. Why did Kaguya fall off after they sealed the deal, or as IRyS put it, "connected?" Because there were NO MORE STAKES LEFT, the tension dropped unless you cared about the "father is against us!" stuff. Why did Higehiro work out? The premise is BUILT ON TENSION!
Tension is what makes people love a story. The best romance stories play out like trying to put two magnets together from matching poles: The repelling tension builds and builds, growing the anticipation, the force, the desire to see it just, finally, CLICK, until the pressure is too much, and the magnet flips to the other end and locks into place!