Monica and Chandler were a couple for the majority of Friends' run, embodying many of the fantasies of what it would be like to fall in love with your best friend. You already know all each other's weird crap! It would be so easy! La la la. But as cute a couple as they make, Chandler isn't Monica's great true love. That would be Richard.
Richard's the underappreciated romantic hero of the show — he's this grown-ass adult, basically the poster-man for hunky masculinity (surely Tom Selleck is on a number of actual posters), confident, wise, and kind. He's got the mustache that launched a thousand ships. He was romantic and understanding, he helped Monica stand up to her parents, and he even made up a weird quirk so that Monica wouldn't feel lonely in her neurosis. Of the various non-Friend romantic interests on the show, Richard is by far the most special, and that includes Phoebe's eventual husband, Mike. Mike's nice and all, but he's no Richard. There's a reason Richard's reappearance in "The Proposal" felt significant rather than like a cheap plot twist, and that was that Richard really was still a contender. They have an enduring love! Their relationship is fundamentally unresolved; in a parallel universe, Monica choses Richard and lives happily ever after churning out hirsute babies and getting tender eye exams.
Ultimately, Monica made the right choice: She wanted a certain kind of life, and she settled down with the person who wanted it, too. This is perhaps the most realistic aspect of the entire show — you really can Yenta yourself into a solid relationship by thinking about marriage as a partnership, not just a romance. "The One" isn't a person, it's a relationship, and that involves shared goals and good timing and a mutual support network, ways Chandler is certainly a better practical match. They have their own kind of joyous, jokey romance, and as the show demonstrates over and over, Chandler's devotion to Monica and his desire to live up to her standards are profound and unflagging. It's all very sweet. But as Monica said herself, Chandler will always be the man who peed on her. And he can't even grow a mustache.
Did no one tell you life was gonna be this way? We're counting down the days until Friends is on Netflix.