In Clash of Kings, why did Theon seize Winterfell? Robb and Theon were great friends. Is it mostly because of the loyalty to his father?
It was not loyalty but ambition and the desire for approval from his father.
Theon had come to the Iron Islands expecting to be treated as the heir and to forge an alliance with the Starks, but his father saw him as soft and estranged from his people's customs. Allying with the Starks was not acceptable to Balon, who humiliated Theon, then sent him to do minor raids, while his sister Asha got the much bigger and more important command.
Conquering Winterfell was not what he was sent to do, it was his idea to do something exceptional that would prove him a worthy heir after all.
Theon suffered a multi-layered problem. He expected that he would be welcomed back as heir of his father and accorded with all the due respect that he was accustomed to in the Greenlands as per his birth and station but he was away for so long, he forgot how Ironborn were different from the Greenlands lordlings he grew up with.
When he came to Iron Islands, his uncle noticed that Theon was reluctant to kneel and receive his blessings, mindful of his nice clothes.
The ground was all stones and mud. "Uncle, I—"
"Kneel. Or are you too proud now, a lordling of the green lands come among us?"
ACOK - Theon I
The Priest also told him that he shouldn't take it for granted that he would inherit the Seastone chair.
"If my father makes plans for war, I must know of them. I am not 'any man,' I am heir to Pyke and the Iron Islands."
"As to that," his uncle said, "we shall see."
The words were a slap in the face. "We shall see? My brothers are both dead. I am my lord father's only living son."
"Your sister lives."
Asha, he thought, confounded. She was three years older than Theon, yet still . . . "A woman may inherit only if there is no male heir in the direct line," he insisted loudly. "I will not be cheated of my rights, I warn you."His uncle grunted.
"You warn a servant of the Drowned God, boy? You have forgotten more than you know. And you are a great fool if you believe your lord father will ever hand these holy islands over to a Stark. Now be silent. The ride is long enough without your magpie chatterings."
ACOK - Theon I
Theon being the idiot that he was, threatened Damphair about his rights. He could have made pious noises about how the Drowned God's chosen King should be a King and Damphair would have supported him. Damphair privately believed Balon was wrong in considering Asha his heir and that the King should be a male. Seeing what he saw of his nephew, Damphair pinned his hopes on his brother Victarion instead.
Then Theon met his father and formed a very bad first impression on him as well, making him believe that Starks had made him weak and soft, unfit to lead the Ironborn.
Theon touched the gold chain. He had forgotten. It has been so long . . . In the Old Way, women might decorate themselves with ornaments bought with coin, but a warrior wore only the jewelry he took off the corpses of enemies slain by his own hand. Paying the iron price, it was called.
"You blush red as a maid, Theon. A question was asked. Is it the gold price you paid, or the iron?"
"The gold," Theon admitted.
His father slid his fingers under the necklace and gave it a yank so hard it was like to take Theon's head off, had the chain not snapped first. "My daughter has taken an axe for a lover," Lord Balon said. "I will not have my son bedeck himself like a whore." He dropped the broken chain onto the brazier, where it slid down among the coals. "It is as I feared. The green lands have made you soft, and the Starks have made you theirs."
"You're wrong. Ned Stark was my gaoler, but my blood is still salt and iron."
Lord Balon turned away to warm his bony hands over the brazier. "Yet the Stark pup sends you to me like a well-trained raven, clutching his little message."
"There is nothing small about the letter I bear," Theon said, "and the offer he makes is one I suggested to him."
"This wolf king heeds your counsel, does he?" The notion seemed to amuse Lord Balon.
"He heeds me, yes. I've hunted with him, trained with him, shared meat and mead with him, warred at his side. I have earned his trust. He looks on me as an older brother, he—"
"No." His father jabbed a finger at his face. "Not here, not in Pyke, not in my hearing, you will not name him brother, this son of the man who put your true brothers to the sword. Or have you forgotten Rodrik and Maron, who were your own blood?
ACOK - Theon I
Here Theon showed himself unworthy by wearing ornaments bought with the gold price and acting as a glorified raven for Robb. He also made the mistake of naming Robb brother-like, which infuriated Lord Balon.
So that's the background, Lord Balon didn't trust Theon and feared that his last living son had turned soft. So when he assigned tasks to his Captains, he gave the mean task of Reavers' work to Theon and added Dagmer Cleftjaw and Aeron Damphair to his advisers, which essentially turned Theon's command to a nominal one. The two were supposed to observe Theon and make reports to Balon of their assessment. Theon was infuriated that he was being treated as such and felt wasted, especially as Asha was going to play a bigger role.
His thrice-damned sister was sailing her Black Wind north even now, sure to win a castle of her own. Lord Balon had let no word of the hosting escape the Iron Islands, and Theon‟s bloody work along the Stony Shore would be put down to sea raiders out for plunder. The northmen would not realize their true peril, not until the hammers fell on Deepwood Motte and Moat Cailin. And after all is done and won, they will make songs for that bitch Asha, and forget that I was even here. That is, if he allowed it
ACOK - Theon III
Then he had his talk with Dagmer Cleftjaw:
“If I had a man like you in my service, I should not waste him on this child's business of harrying and burning. This is no work for Lord Balon's best man.”
Dagmer's grin twisted his lips apart and showed the brown splinters of his teeth. “Nor for his trueborn son?” He hooted. “I know you too well, Theon. I saw you take your first step, helped you bend your first bow. 'Tis not me who feels wasted.”
“By rights I should have my sister's command,” he admitted, uncomfortably aware of how peevish that sounded.
“You take this business too hard, boy. It is only that your lord father does not know you. With your brothers dead and you taken by the wolves, your sister was his solace. He learned to rely on her, and she has never failed him.”
“Nor have I. The Starks knew my worth. I was one of Brynden Blackfish‟s picked scouts, and I charged with the first wave in the Whispering Wood. I was that close to crossing swords with the Kingslayer himself.” Theon held his hands two feet apart. “Daryn Hornwood came between us, and died for it.”
“Why do you tell me this?” Dagmer asked. “It was me who put your first sword in your hand. I know you are no craven.”
“Does my father?”
The hoary old warrior looked as if he had bitten into something he did not like the taste of. “It is only . . . Theon, the Boy Wolf is your friend, and these Starks had you for ten years.”
“I am no Stark.” Lord Eddard saw to that. “I am a Greyjoy, and I mean to be my father's heir. How can I do that unless I prove myself with some great deed?”
ACOK - Theon III
So there you have it.
- Theon knew his father and uncle didn't trust him and thought he was a Stark inside. They feared he had given up on the Ironborn ways.
- Theon was aware that his sister was a contender for their father's chair.
- Asha's command had more men and more prestigious targets while Theon was given Reavers' work. How was he to prove that he was better than Asha if he kept following orders?
- He wanted to prove his loyalty and his worth.
So Winterfell gave him a chance to do all of these in one stroke. By capturing Winterfell with a handful of men, especially when his father expected Winterfell to defy them for years:
- Theon would prove that he had abandoned House Stark forever and was loyal to House Greyjoy.
- He would prove that he still held Ironborn ways of conquest dear.
- He would prove that he was worthy by taking a castle expected to defy them for years with a handful of men.
There's no evidence in the books that Theon was jealous of Robb. In the show he's definitely jealous, but not in the books. In fact, there's more evidence that Robb looked up to Theon (for whatever reason), then the other way around.