How did the Justin Bieber & Selena Gomez love story happen?

Justin Bieber and his gal Selena Gomez showed some slight PDA while in the crowd at the Billboard Music Awards. The young couple posed for pics, but cameras also caught Selena with her hand on the teen heartthrob's knee. Justin Bieber and Selena Gomez both won big at the Teen Choice Awards 2011, but