Y'know what? Although this needs to be said (but has been said many times before) understanding is a two way street. So here it is from the other side.
You're alone. You've been alone for years. Sure, you have friends, maybe a lot of them, but those pesky human impulses want more. Everyone around you has found someone. Everyone keeps saying there's nothing wrong with you, but the evidence for this claim is scarce.
You meet someone. You like her (or him, if you're that way inclined) she seems to like you. Hard to tell, seeming as everyone you ask for advice peddles out useless cliches like "if it's meant to happen, it'll happen" and "someone for everyone". After building up the courage and figuring out how to do it, a hard task for someone with your lack of experience, you admit your feelings, and she doesn't feel the same.
You're heartbroken. Your seemingly anual (or even less frequent) singular chance has came and went, and you don't even know why. You talk to friends sooner or later, because hell, you can't keep this kind of hurt to yourself forever. More cliches, somehow even more useless than before.
Spending time with the person you liked is hard enough. Suspicions run through your mind, or simple excuses not to be there. She doesn't want you there. The very sight of you disgusts her. Even a bad reason for it to have happened is better than no reason at all. Eventually she gets someone else, and it becomes impossible to spend time with her, not because of jealousy necessarily, but you'll be looking at that person and using him as a battering ram against your own self esteem.
Now, what I'm saying here isn't that it's right, or fair, that the supposed "victim" of "the friend zone" does this, abandons what was a friend, but that it's understandable. People forget how hard it is to stay around someone you love that only likes you.
And I know why it happened to me. I know every damn day I look in the mirror and somehow look more of an abomination than the day before. Everyone's shallow, I know that, even I'm shallow a little. You can't lie and say that isn't the reason, because 90% of the time, it is. Now I don't do that to girls anymore. I don't disappear with barely a word. Because I don't ask girls out anymore, because I know there is only one conclusion, that she rejects me, and either I leave her forever, or completely destroy myself by staying. I've accepted the fact that I will never be wanted. I am a horrifying, disgusting creature, and it was always a bit unfair to expect anyone to be attracted to me, but that doesn't make it an easy thing to accept.
So yeah. It's not fair or right that this "friendzone" effect exists, but it does, because you just don't stop wanting someone because they don't want them back. Quoted because I'm interested to know what you think.