I dunno if the idea of "the one" is just a romantic notion that people love to believe in cus then they can hope on that they'll find someone right for them at some point in their lives. Or the idea of "the one" could be true.
I do believe that there's someone out there for everyone, but that may not be limited to just one person. What makes "the one" "the one", someone once told me, is that that person cannot be replaced because of the moments and memories that you two share. Those are uniquely linked to the person with whom you built those memories, and that makes them special.
And recently I think I've found out when, in my eyes anyway, you know that you should marry someone. I think it's when you feel and want to be truly one with that person and for them to be rightfully yours, and that "rightfully" is a different, deeper kind than the right you have in a relationship. When their body and life and soul and love are rightfully yours and never to be anyone else's because you alone have the privilege of that. I could be very wrong here, but it's just a conclusion I've made, for now.
I've often wondered why you love someone. What makes them special, what makes you care, even when they've stopped caring?