Probably for the same reason that crowds go nuts for guitar solos that feature the same note played over and over.

See: Clapton, Eric

Sent by Adam | 11:34 AM ET | 03-13-2008

I think because it's played with your mouth, so it's like an extention of a singer's voice. And there's an intimacy associated with it; you usually think of a harmonica played for a small group of people and not a stadium. And it's a "common" instrument; people seem to go crazy for kazoo-playing too.

Sent by Jason Viola | 11:54 AM ET | 03-13-2008

i think it's the simplicity of its soul, its present moment magic and surrender

Sent by Jehan | 11:51 PM ET | 03-13-2008

Jeff Tweedy made a comment about that when I saw him. He was playing "She's a Jar," which features a 6 or 7 note harmonica line that sounds like it was plucked from "How to Play Harmonic - Beginner Level." Every time I've seen him the crowd goes absolutely nuts when he plays it though. So this time he stopped and commented that his harmonica line is not and never has been worthy of applause. At least he admits it, still a great song though.

Sent by John Michael | 11:15 AM ET | 03-14-2008

Kazoo is even more fun.

Sent by Tom Hendricks | 11:01 PM ET | 03-28-2008

The harmonica, even played sloppily, gives a third line to a band that is guitar/vocals based. I enjoy that about it.

Sent by Roy A. Justus | 3:55 AM ET | 06-20-2008