That's not a cool thing to do to your friend. Of course if he's getting a lot of ass he'll want to tell his friend about it. He's not doing it in a way to rub it in your face and make you feel like crap, is he? Although I know when other people talk about all the ass they get I feel inadequate in comparison, even if this was not their intention (sometimes it is though). But you were trying to make this guy feel bad

What you can do is tell him you're sorry for being a dick and sometime later tell him you don't like hearing about all his "conquests" all the time because it's not something you can share in. It makes for a really lop-sided discussion, I know because when I was home over winter break I heard a lot of these fairytale stories but couldn't contribute

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"Hey Nick, wanna go lolerskating later?"

"Sorry I can't, I booked tickets for a ride on the roflcopter today"