Yeah, you're not the only one it's happened to.

Logically, it makes complete and utter sense. Most common people like you or me for instance (despite my weirdness, there is something normal beneath the layers of abnormality), want to be with someone you can be completely comfortable with, a person that you can get to know very well and who has at least a few things in common, or a basic understanding of you. So basically, that's describing some friend you've had for years and only really considered in the platonic way. Yeah sure, maybe once or twice you might have considered them in THAT way, but not seriously.

So this is how it works: Your life is completely fine and then BAM! A dodgy break up or mid-life crisis happens and you find yourself crying on the shoulder of your suddenly potential more-than-friend saviour who wants nothing more than for you to recover and be all happy again. What do you do? You stick to them like glue, of course. Nothing better than a person who would willingly sit with you as you get all that bad stuff out, or better yet, maybe even help you. Then it hits you like a brick in the face: Dear Lord, I am in love with my best friend in more than a normal way.

Or maybe it happened a different way?

Another popular one: You're watching TV one day and you're sat with your mate, who you've known for ageeees. Somehow, your brain switches from movie-mode to that of personal reflection. You reflect on the fact that those feelings you always assumed to be completely standard-friendship stuff could be more. All that respect you have for each other, all those things you share in common, all those things you don't, all those funny times like when you de-kegged that dude who thought he was a hardguy and you both laughed like maniacs for ages afterwards, they all add together to make something that seems a great foundation for a powerful relationship. And yet all it starts from is one simple moment of realisation. This is how I imagine a scenario based loosely on that structure would work out:

*Quick glance over at friend sat next to him*
(Internal thought) Whoa, she's a lot prettier than I remember her being when we used to play footy all those years ago.

And then BAM! That feeling appears inside you, which makes you realise you might possibly be sub-consciously aiming for more than friendship in the future cards. Some people go for it, some people are too scared to ruin what they already have but all I can say is that it depends on who you are and how close you are to your potential soulmate/boyfriend/girlfriend/acquintance/sidekick.

Can it be stopped? Not really. You could possibly endure something close to torture if your feelings aren't reciprocated and it will feel like you are being stabbed repeatedly in the heart (slightly melodramatic?), but sometimes it's best just to face the music instead of hiding the change in your feelings and dealing with the cards that have been dealt, cause they could potentially work in your favour.

So my answer is: I'm sorry, but there is absolutely nothing I can do to help you with your predicament. I've been there before and apart from considering memory erasing (watch Eternal Sunshine Of The Spotless Mind for more information...) there is jack all I can do for you. Sorry.

But I can say it loud and proud because I feel your frustration: Help! I'm falling in love with my best friend!