My son Eli (14) and I went to see I am Legend this afternoon. I'm not here to review the movie, although I'll say Will Smith and his dog are ++++ and the zombies are meh.
Throughout the movie you hear Bob Marley's "Three Little Birds." That's the one that goes, "Don't worry/'Bout a thing/Cause every little thing's/Gonna be all right." At some point he tries to explain why Marley is Great to the first non-infected human being (Alice Braga) he's encountered in over 3 years. She says, "Damien?" (Marley's son, of course. And all of a sudden you really begin to appreciate the age difference between the two characters.)
So after the movie I think to educate Eli about Marley. He says, "Oh, yeah, that was that "Don't worry, be happy," song, right?
Um, no. Bobby McFerrin has considerable talent, but he's no Marley. And they're not the same. Not at all.
Try playing both songs, back to back, to appreciate the difference. Even Eli, in his oblique way, acknowledged that there might be some differences between the songs. And Eli HATES to admit that he's wrong.