It all started with that notorious Street Fighter III: Third Strike video. Innuendo Studios developer Ian Danskin got interested in watching competitive fighting games, then got interested in the EVO tournament, and ... before he knew it ... he was a full-blown fan of professional Smash Bros. play.
In the video above, he explains this in an entertaining and sensible way: The way eSports fandom escalates, what it shares with traditional sports, and how Smash Bros., specifically, is engaging to watch as a competitive game.
If you don't understand the appeal of eSports, this video should help; if you want to understand what makes for a compelling game to watch because you're making one, it should help with that too.