Writers create the worlds that their characters live in, but they don't always treat them the way that they deserve. Why do you think fanfiction is so popular? It's because viewers need to fix the mistakes that the writers made to their dearest characters. Those viewers think that they can do the stories justice, or at least pretend that the ending in their head is the one that actually occurred on the show.
Even when a series isn't centered on relationships, couples are bound to pop up in the storylines. After all, forming tight bonds with others is what makes us human. It's rare to see a show without any married couples or hookups, because they're a major source of conflict in the real world. Watching characters we love go on dates and fall in love helps you connect to them, because you've been in the same position and can relate.
However, while breakups and divorces in real life can't be controlled, your favorite television shows can be. That's why it's so upsetting when a perfect couple is written to break up. It's as if the writers, whom you've trusted to give you a fulfilling story, betrayed you.
Of course, even though the writers were in charge of creating that final episode, you can pretend it never happened. That's the beauty of fiction...