Are you really not understanding the type of bond that can be created between two people in a long-term committed relationship?
erplexed If it's a good relationship it's often hard to forget the person, let alone just pick up and walk away. Low self-esteem or self-worth isn't always the reason why someone has a hard time leaving.
OP already said her friend thought he was the one, thought that they were going to get married and have a future together. How do you go from loving someone and planning your future with him to walking away point blank with no discussion or anything? Then let's not forget the denial. You sound like a crazy person. Have you ever been in a long-term (serious/good) relationship?
And to the bolded I seriously hope you don't, at least not on this board, because the "advice" I give will be just as rude and nonsensical as yours.