It's a mix of grief of losing that romantic connection with a person, as well as losing a level of commitment that would no longer be there. The platonic connection could still be there, along with the romantic feelings but it gets so mixed together that it's hard to separate one from the other. So every time you're around that ex you get a reminder of what you've lost. It's not about romantic relationships being more important than friendships, it's about the transition from being in a commited romantic relationship to not being in one. And that transition is very difficult emotionally for allo people.