Rin Little said:
So long story short, I've recently started dating a guy who I met and he's awesome. We always enjoy spending time together, we're getting to be very close and we have a lot in common. Everything's going great...
But one of my guy friends is straight-up being an outright ass almost. When I first mentioned that I was dating someone new his first reaction was "I'm happy that you're happy, but I know that I could make you happy too" and launches into this whole sob story about why no one hangs out with him. I'm not very close with this guy at all, but he's constantly telling me how attracted he is to me and all this other shit and he barely even knows me because any time I saw him in person he just sat in a corner and didn't talk to anyone. Just a little bit ago when he was talking to me on IM he mentioned that he was feeling really horny... Why the fuck would I want to know that?! Seriously, this guy doesn't know shit about me and yet he's making all these comments as if he could convince me to dump my boyfriend and instead date him when he doesn't have a real job and no car and constantly whines about crap...
Why do guys pull this shit?!
You have the most personal insight into this person, normally I am fairly amazing at breaking down a person personality and reason, but it's more a face to face thing, or piles of information about them.
OT: The guy sounds stuck between a geek and a horny teenager, my guess is he is not the most social at first due to people being strangers, but after that short stage he becomes fairly open. This I think will cause him to seem to friendly to certain people, you I assume will have a longer time needed to class someone as a friend or a close friend, and this guy ofc is being friendly to early to you.
The guy also sound jealous, and fairly egotistical. I guess because he isn't the most open to people, he hasn't been attacked much for being egotistical. (I am unsure if I am making sense XD I basically am trying to say he hasn't learnt to not be egotistical

)
It's seem the issue is his personality is partly egotistical, and I guess you haven't told him off in anyway.
He due to his social standing does get much, which is why he is fairly desperate with you, attempting to act overly confident, however just acting like a creep.
Hope this help, and my sleep deprivation hasn't made this nonsensical.