"There have been too many mistakes where Harry Potter is concerned. Some of them have been my own. That Potter lives is due more to my errors than to his triumphs." (1.32)
Voldemort here is certain that everything that happens is up to him – and nobody else. The only choices that matter in his mind are his own, a belief that really betrays his arrogance.
"Has anyone ever tried sticking a sword in Voldemort? Maybe the Ministry should put some people on to that, instead of wasting their time stripping down Deluminators or covering up breakouts from Azkaban. So this is what you've been doing, Minister, shut up in your office, trying to break open a Snitch? People are dying – I was nearly one of them – Voldemort chased me across three counties, he killed Mad-Eye Moody, but there's been no word about any of that from the ministry, has there? And you still expect us to cooperate with you!" (7.115)
Here, Harry really seizes the opportunity to lambast the Minister of Magic, Rufus Scrimgeour, for not taking action – it's clear that Harry's decision to act on his own is far removed from the official stance of the Ministry.
"I – I made a grave mistake in marrying Tonks. I did it against my better judgment and I have regretted it very much ever since… Don't you understand what I've done to my wife and my unborn child? I should never have married her, I've made her an outcast!" (11.64-66)
Lupin is tortured by the idea that he might have already ruined his unborn child's life, simply through his werewolf nature. However, Harry reminds him that a choice that's been made cannot then be unmade – after all, Lupin married Tonks because he loves her, and there's nothing he can do to get rid of that bond.
"I hate it, I hate the fact that [Voldemort] can get inside me, that I have to watch him when he's most dangerous. But I'm going to use it."
"Forget Dumbledore. This is my choice, nobody else's." (12.58)
Harry's finally started to act totally based on his own judgment – he realizes that nobody else's decisions or instincts are any more correct than his own, and he's wholly responsible for how he handles his connection to Voldemort, which is, after all, just between the two of them.
"… Of course, the manner of taking matters. Much depends on the wand itself. In general, however, where a wand has been won, its allegiance will change."
There was silence in the room, except for the distant rushing of the sea.
"You talk about wands like they've got feelings," said Harry, "like they can think for themselves."
"The wand chooses the wizard," said Ollivander. (24.115-117)
Interesting… so, not only humans make choices here, but so do their wands. This is a fascinating twist on this theme that will prove to be fundamental to the plot's resolution. It's another way in which Harry's approach to magic differs from Voldemort's; while the latter thinks that he can force anything he wants to happen, Harry realizes that some things (like the Elder Wand choosing him) happen out of their own free will.
As he followed Bill back to the others a wry thought came to him, born no doubt of the wine he had drunk. He seemed set on course to become just as reckless a godfather to Teddy Lupin as Sirius Black had been to him. (25.56)
Harry recognizes that his choices have been reckless and dangerous – and that he's following a pattern he's been taught by Sirius. However, we know that Harry's a very different man from his godfather… and, as much as we love Sirius, a wiser one overall.
And Dumbledore had known that Harry would not duck out, that he would keep going to the end, even though it was his end, because he had taken trouble to get to know him, hadn't he? Dumbledore knew, as Voldemort knew, that Harry would not let anyone else die for him now that he had discovered it was in his power to stop it. (34.7)
Dumbledore knew Harry's character, and thus knew what choice the young man would have to make. He used this knowledge of Harry to ensure that his plans would fall into place, making it difficult to figure out if this is predetermined by Dumbledore, or a choice freely made by Harry, or both.
"I am sorry too," said Lupin. "Sorry I will never know him – but he will know why I died and I hope he will understand. I was trying to make a world in which he could live a happier life." (34.57)
Lupin, here speaking of his son Teddy, is sure about his choice to fight, even though it means that he'll never get to know his only child. He speaks with conviction, knowing that he did all he could to help build a future.
"But…" Harry raised his hand instinctively toward the lightning scar. It did not seem to be there. "But I should have died – I didn't defend myself! I meant to let him kill me!"
"And that," said Dumbledore, "will, I think, have made all the difference." (35)
Ah… so, we see, it's not Harry's actual demise, but his decision to die that counts. This choice, made of his own free will, and for the sake of others, is what saves him in the end.
"One of us?" jeered Voldemort, and his whole body was taut and his red eyes stared, a snake that was about to strike. You think it will be you, do you, the boy who has survived by accident, and because Dumbledore was pulling the strings?"
"Accident, was it, when my mother died to save me?" asked Harry. […] "Accident, when I decided to fight in that graveyard? Accident, that I didn't defend myself tonight, and still survived, and returned to fight again?"
"Accidents!" screamed Voldemort, but still he did not strike. […] "Accident and chance and the fact that you crouched and sniveled behind the skirts of greater men and women, and permitted me to kill them for you!" (36.95-97)
Yet again, Voldemort makes the fatal error of underestimating the choices and deliberate actions of others, assuming that only his choices matter.
"Albus Severus," Harry said quietly… "you were named for two headmasters of Hogwarts. One of them was a Slytherin and he was probably the bravest man I ever knew."
"But justsay –."
"– then Slytherin House will have gained an excellent student, won't it? It doesn't matter to us, Al. But if it matters to you, you'll be able to choose Gryffindor over Slytherin. The Sorting Hat takes your choice into account."
"It did for me," said Harry. (Epilogue.38)
Harry assures young Albus that his choice does matter – and that he can make his own fate, a lesson that we should all take away from Harry's story!