Thanks for this great topic.
Great answers and points made by all of you and with this said I truly believe everyone who plays live gigs are in somewhat of a brotherhood. Case in point, say you point out any of the reasons or points made above to a civilian/non-musician, they just won't get it. But when you say feed off of an audience, connect with all or just a portion of that audience, the music bringing back memories (either good or bad) to members of that audience, or as Notes says just fun to another gigging musician he/she gets it 100%!
I think Billy Joel's "The Piano Man" is the tune that most gigging musicians can relate to simply because we have all been there, period. It just might be the Gigging Musician's Anthem, especially the ones who have played the small venues around the country.
Again, good topic!