what makes a word popular?

Sometimes I hear certain new words in my friends' or acquaintances' lexicon, words that have suddenly become popular, and I wonder how or why they've become so common? Why, all of a sudden, is everyone I know saying "doppelganger" "napalm" or "gravy" when those words didn't even exist in their vocabulary five months ago? (And yes, those are the actual words that have inspired this post...)

popular culture

This is what I've come up with: Pop culture. Yeah, pop culture. Seems obvious, right? So far, people in their 20's and 30's seem to be the main-abusers of the above words, and I figure the major reasons are because we're watching the same shows, reading the same magazines, and viewing similar commentary from friends on social networking sites, like Facebook. You ever read an email from someone and then reply using a word that you haven't used before thinking to yourself "I'm so clever," only to realize that that same word was right there in your friend's email? I have. I revise and delete when that happens so my friend doesn't think I'm an echo, but then I'll use the word in a future email to someone else (sometimes without even thinking), thus creating a chain of popular vocabulary.

popular words, man

A dear friend of mine says "man" a lot. As in, "I worked on this project today that was really tough, man." or "I don't know, it's one of those tricky things, man." After hearing her say this several times, I now find myself using "man" much more frequently...man.

East Coast vs. West Coast

Maybe it's regional? Californians use some interesting slang (like man, although I think that started from an East-Coaster?). When I first moved to New York I worked with people from nearby towns associated with heavy, nasal accents - Long Island, the Bronx, Brooklyn and New Jersey. When I started saying "Bless" after someone sneezed, instead of "Bless you", and pa-jahmas instead of pah-jamas (as I'd been saying it) I realized that my transition into a New Yorker had officially begun...it's since reversed.

what about you?

So why do you think some words suddenly become popular? Is it pop culture? Regional? Age? What are some of the popular words that you currently hear, and where are they being used?