1. He's stuck in 1999 and only has a beeper and beepers don't store phone numbers. I'll start with this one because it's really, truly, the most unlikely reason. Yet, when you are running through different potential play-by-plays as to why someone didn't ask for your number (or call you, or ask you on a date, etc.), you'll always, without a doubt, start with the most absurd possibility.
2. In serendipity he trusts. The ol' "if it's meant to be, then we will of course run into each other again at some undisclosed location, in one of the four boroughs, before winter kicks our butts back into hibernation."
3. Maybe he's not fully convinced you're into him. Your "resting" face can be misleading and though your heart is pumping gimme gimme more, you're face is all like, "I've had one too many glasses of merlot and even though my bright eyes are closing, I'm all here with ya, babe."
4. He's too nervous. It can be that simple, really.
5. He has a girlfriend and/or a boyfriend. Mazel Tov to them.
6. He's just not interested in ever seeing you again. This one's a bit ugly to digest.
7. Three is such an overwhelming and defunct crowd. You, him, your sorority sister.... while this poor lad is struggling to spark your interest with amassing conversation, he also has to give some love to your gal pal. The nucleolus of dirty looks from her eyes shoot threats as bold as don't you dare break her heart or I'll tell my 5,489 Twitter followers what a baboon you are.
8. You didn't give him yours. Oh wait, you didn't know this was an option, did you? There's no footnote in your self-help dating bible that reads, "If you like a guy, YOU should do something about it?" Give him your digits, your AIM screen name, your email address. Whatever you're comfortable with. If he contacts you, he contacts you. If he doesn't, well, please see #1.