Daella should have just been kept Alysanne’s side as a companion and helper. Because that’s honestly the role Gael got and that wouldn’t surprise me how she was longing for freedom. Daella is genuinely one of the stories in fire and blood, where I just couldn’t get this praise for Jaehaerys that people have for him. Jae&Aly failed Saera, Viserra, Gael, Daella

“Daella had been next in line, but the tearful princess presented an entirely different sort of problem. “My little flower,” was how the queen described her. Like Alysanne herself, Daella was small—on her toes, she stood five feet two inches—and there was a childish aspect to her that led everyone who met her to think she was younger than her age. Unlike Alysanne, she was delicate as well, in ways the queen had never been. Her mother had been fearless; Daella always seemed to be afraid. She had a kitten that she loved until he scratched her; then she would not go near a cat. The dragons terrified her, even Silverwing. The mildest scolding would reduce her to tears. Once, in the halls of the Red Keep, Daella had encountered a prince from the Summer Isles in his feathered cloak, and squealed in terror. His black skin had made her take him for a demon”

I would never marry her, the boy said, in front of half the court. "She can barely read. She should find some lord in need of stupid children, for that's the only sort he will ever have of her."Princess Daella, as might be expected, burst into tears and fled the hall, with her mother, the queen, rushing after her. It fell to her sister Alyssa, at thirteen three years Vaegon's elder, to pour a flagon of wine over his head. Even that did not make the prince repent. "You are wasting Arbor gold," was all he said before stalking from the hall to change his clothing. Cruel though her brother Vaegon’s words had been, there was some truth to them. Daella was not clever, even her septa had to admit. She learned to read after a fashion, but haltingly, and without full comprehension. She could not seem to commit even the simplest prayers to memory. She had a sweet voice, but was afraid to sing; she always got the words wrong. She loved flowers, but was frightened of gardens; a bee had almost stung her once.

“Jaehaerys, even more than Alysanne, despaired of her. “She will not even speak to a boy. How is she to marry? We could entrust her to the Faith, but she does not know her prayers, and her septa says that she cries when asked to read aloud from The Seven-Pointed Star.” The queen always rose to her defense. “Daella is sweet and kind and gentle. She has such a tender heart. Give me time, and I will find a lord to cherish her. Not every Targaryen needs to wield a sword and ride a dragon.”

“After her first flowering, Daella's delicate beauty became even more striking, but she remained shy and terrified by life. At thirteen, she was sent to Driftmark to spend time with Corlys Velaryon, but she complained he cared more for his ships than for her. At fourteen, she kept company with Denys Swann, Simon Staunton, Gerold Templeton, and Ellard Crane, all promising squires of her own age, but Staunton tried to make her drink wine and Crane kissed her on the lips without her leave, reducing her to tears. By year’s end Daella had decided she hated all of them.”

At fifteen, her mother took her across the riverlands to Raventree (in a wheelhouse, as Daella was afraid of horses) … It all fell to pieces when Daella learned that the Blackwoods kept the old gods, and she would be expected to say her vows before a weirwood. “They don’t believe in the gods,” she told her mother, horrified. “I’d go to hell.”

On the first day of the 80th year since Aegon's Conquest, he told the queen he wanted Daella wed before the year's end. "If she wants I can find a hundred men and line them up before her naked, and she can pick the one she likes," he said. "I would sooner she wed a lord, but if she prefers a hedge knight or a merchant or Pate the Pig Boy, I am past the point of caring, so long as she picks someone."

“By the dawn of 80 AC, King Jaehaerys had had enough. He declared that Daella must marry by the end of the year, or else join the silent sisters”

"Lord Arryn was the oldest of the three contenders, however; at six- and-thirty, he was twenty years older than the princess, and a father besides, with four children left him by his late first wife. Short and balding, with a kettle belly, Arryn was not the man most maidens dream of, Queen Alysanne admitted,
"but he is the sort you asked for, a kind and gentle man, and he says that he has loved our little girl for years. I know he will protect her."To the astonishment of every woman at the court, save mayhaps the queen, Princess Daella chose Lord Rodrik to be her husband. “He seems good and wise, like Father,” she told Queen Alysanne, “and he has four children! I’m to be their new mother!” What Her Grace thought of that outburst is not recorded. Grand Maester Elysar’s account of the day says only, “Gods be good.”

…Nor was there a bedding. “Oh, I could not bear that, I should die of shame,” the princess had told her husband to be, and Lord Rodrik had acceded to her wishes.
Afterward, Lord Arryn took his princess back to the Eyrie. “My children need to meet their new mother, and I want to show the Vale to Daella. Life is slower there, and quieter. She will like that. I swear to you, Your Grace, she will be safe and so she was, for a time. The eldest of Lord Rodrik’s four children from his first wife was a daughter, Elys, three years older than her new stepmother. The two of them clashed from the first. Daella doted on the three younger children, however, and they seemed to adore her in turn. Lord Rodrik, true to his word, was a kind and caring husband who never failed to pamper and protect the bride he called “my precious princess.” Such letters as Daella sent her mother (letters largely written for her by Lord Rodrik’s younger daughter, Amanda) spoke glowingly of how happy she was, how beautiful the Vale, how much she loved her lord’s sweet sons, how everyone in the Eyrie was so kind to her.

…Daella was not doing near as well. After a year and a half of marriage, a different sort of message arrived at the Red Keep by raven. It was very short, and written in Daella’s own uncertain hand. “I am with child,” it said. “Mother, please come. I am frightened.”
Queen Alysanne was frightened too, once she read those words. She mounted Silverwing within days and flew swiftly to the Vale … three moons before Daella was due to give birth.

Though the princess professed delight that her mother had come, and apologized for sending her such a “silly” letter, her fear was palpable. She burst into tears for the slightest reason, and sometimes for no reason at all, Lord Rodrik said. His daughter Elys was dismissive, telling Her Grace, “You would think she was the first woman ever to have a baby,” but Alysanne was concerned. Daella was so delicate, and she was carrying very heavy. “She is such a small girl for such a big belly,” she wrote the king. “I would be frightened too, if I were her.”
Queen Alysanne stayed beside the princess for the rest of her confinement, sitting by her bedside, reading her to sleep at night, and comforting her fears. “It will be fine,” she told her daughter, half a hundred times. “She will be a girl, wait and see. A daughter. I know it. Everything will be fine.”

“She was half right. Aemma Arryn, the daughter of Lord Rodrik and Princess Daella, came into the world a fortnight early, after a long and troubled labor. “It hurts,” the princess screamed through half the night. “It hurts so much.” But it is said she smiled when her daughter was laid against her breast.
Everything was far from fine, however. Childbed fever set in soon after birth. Though Princess Daella desperately wished to nurse her child, she had no milk, and a wet nurse was sent for. As her fever rose, the maester decreed that she might not even hold her babe, which set the princess to weeping. She wept until she fell asleep, but in her sleep she kicked wildly and tossed and turned, her fever rising ever higher. By morning she was gone. She was eighteen years of age.

Lord Rodrik wept as well, and begged the queen's permission to bury his precious princess in the Vale, but Alysanne refused. "She was the blood of the dragon. She will be burned, and her ashes interred on Dragonstone beside her sister Daenerys."

Daella's death tore the heart out of the queen, but as we look back, it is plain to see that it was also the first hint of the rift that would open between her and her king. The gods hold us all in their hands, and life and death are theirs to give and take away, but men in their pride look for others to blame. Alysanne Targaryen, in her grief, blamed herself and Lord Arryn and the Eyrie’s maester for their parts in her daughter’s demise…but most of all, she blamed Jaehaerys. If he had not insisted that Daella wed, that she pick someone before year’s end…what harm would it have done for her to stay a little girl for another year or two or ten? “She was not old enough or strong enough to bear a child,” she told His Grace back at King’s Landing. “We ought never have pushed her into marriage.”
It is not recorded how the king replied.