Sue Storm: Why are you trying to destroy us?
Silver Surfer: I don't have a choice.
Sue Storm: You always have a choice.
From Fantastic Four: Rise of the Silver Surfer, when Sue Storm meets the Silver Surfer for the first time. Silver Surfer later shares with Sue that he serves Galactus (a planet-destroying entity) in order to save his own planet and the woman he loves. Ultimately, however, Silver Surfer realizes he does indeed have a choice, even if it's a hard one to make.
(Image: copyrighted by 20th Century Fox; from IGN's site on the film)