In Chamber of Secrets, Professor Dumbledore presents the all-important message of the Harry Potter series: "it is our choices [...] that show what we truly are, far more than our abilities" (18.61). Starting in this novel, we being to realize that though Harry and Voldemort have a lot of life circumstances in common, it is their choices that define them and make them truly different. J.K. Rowling has stated that even the all-important prophecy revealed in Book 5 would not matter if both Harry and Voldemort chose to turn away from each other and not fight. Choice is vital to determining a person's character.
Yet, while we admire this theme, we also realize that there are lots of limitations on people's choices that they can't control. For example, Harry can't choose not to face Voldemort because Voldemort won't let him. Even when Harry isn't looking for Voldemort at Hogwarts, he still encounters him by chance thanks to a cursed diary in Ginny Weasley's hands. So the question of choice versus fate is a bit more ambiguous than Professor Dumbledore's idealistic statement might make it seem.