"Initially, the song began with the words 'Yeah, you really got me now', but that was changed in the studio on the advice of Hal Carter, an impresario who had previously been hired to hone the group's image. Although Carter had been recently dismissed from his position with the Kinks, he did them one last favor by suggesting that Ray Davies add a new first word to the song to make it more personal and direct. Davies settled on replacing 'Yeah' with 'Girl', and recording commenced." (Link courtesy of Flaskaland.)
Update: I've been looking at some other work by Charlotte Robinson, the author of the Kinks article. She's interviewed some very cool people, like Roberta Bayley and Tish and Snooky Bellomo. I never get tired of reading about these people, as cliche as that whole "New York punk" thing is now.