Why do all main leads end up together? Why can’t they just be with the second leads? 

Yongsun has yet to find her main lead counterpart whilst Byulyi believes she’s hers, which undoubtedly would have made Yongsun deem her as second lead. The biggest problem however was Yongsun thinks of Byulyi as her sister—nothing more, nothing less.

Lilly Leery: What's a soul mate?

Dawson: It's uh... Well, it's like a best friend, but more. It's the one person in the world that knows you better than anyone else. It's someone who makes you a better person. Actually, they don't make you a better person, you do that yourself because they inspire you. A soul mate is someone who you carry with you forever. It's the one person who knew you and accepted you and believed in you before anyone else did or when no one else would. And no matter what happens, you'll always love her. Nothing can ever change that. Make sense?

- Dawson (Dawson’s Creek, Season Finale)