Long story short:

Boy meets girl 3 years ago. Tries to hook up with girl, doesn't go further on because she meets another dude during the summer.
Boy doesn't talk with girl for a year because they lose sight of each other.

Boy and girl then reconnect and become best friends, reminiscing on how "they were supposed to go out with each other".
Boy dates girls best friend, girl goes out with another dude.

Both relationship's end, boy and girl cheer both up.

Boy never ever ever EVER felt feelings for this girl (since re-connecting).

However, recently, a friend played a joke saying how much it was obvious we were/should be together. This got me thinking. At first nothing more.

But for the first time, yesterday, I started feeling like I might want it to be real.

And I don't know what to do about it, because I don't feel like she would want it to be real. We've had discussions of us possibly being together in the past, and we've always concluded that we would end up killing each other.

Yet I get a text from her this afternoon about a guy that has been flirting with her and for the first time I feel jealous.

Supply and demand, its a part of our normal nature to want something someone else has.
See a hot girl walkin down the street with a hot guy (in a straight way) and you want her.
See a o.k. girl walkin down the street alone; you just pass her by.
Same girl; but you noticed the difference. For some reason I see relationships go down the drain when someone suggests you should be together, while I dont have it figured out ive learned how to avoid it; but thats not your problem right now its just how you got there.
You need to trust your initial feelings because you are getting a contact high. Put a guy in a forest with a hb6 chick alone for a week; when you come back he is gunna be chasin the crap outa her, even though in a bar he'd tell her no. While for the first week it'll feel great but there will be no LTR because the relationship was based on desperation. If you want a quickie outa her then pursue this but a LTR would be a bad idea.

Lastly NEVER feel jealous; jealousy is the thought that you arn't capable of doing what another simple human being is; you are infinitly capable of anything and with women this should never enter your mind

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To game at your best you have to be willing to do your worst.