Why? Because they are awesome, that's why!  From Deadline:

Twenty million viewers can’t be wrong. Following the warm critical reception and big ratings for Tina Fey and Amy Poehler’s first hosting turn at the Golden Globes earlier this year, the comedy stars have been approached for a second go as emcees of the awards show. Unlike Ricky Gervais’ original deal, which had an option for a second turn as Globes host, I hear Fey and Poehler originally agreed to host this year’s ceremony as a one-time thing with no obligation to come back. But, given their success, it’s no surprise that NBC and Golden Globes producer Dick Clark Prods are asking them to return.

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