Her nightmares didn't always stay the same. Humans were not a static creature, and as a person grew and changed so did their worst fears.
As a little girl, Robin feared not being good enough, for her teachers or her mother. But then came the Buster Call, and she became the Last Oharan. She had been given no choice but to be the best. The legacy of every archeologist to ever be trained at the Tree of Knowledge was placed on her small shoulders. It was a terrible burden to bear, but one she refused to give up. Their confidence in her ability gave her strength, and Robin persevered for twenty long, lonely years.
For a long time after that, her dreams consisted of being chased by a shadow-monster, a consuming void seeking to devour her as she ran and ran and ran, never finding rest. Robin had conquered those dreams by becoming a monster herself, protecting her vulnerable heart with a cloak of darkness. She could be scarier than her enemies, Robin discovered. Monsters preyed on the weak, so she refused to be weak. It was as simple as that, and the transformation from frightened child to Demon of Ohara nearly destroyed her.
Then the Straw Hats found her, adopted her against her will, and Robin dreamed of the day when they would learn to hate her. It was a soul-crushing inevitability, and after learning what it was like not to be alone, she decided she would rather die than go back to the way things were.
That dream faded away with a single shot, her family declaring war on her behalf. Consequences and logic be damned, she was theirs and they were hers. Not even the largest of monsters could stand in the way.
But instead of defeating her nightmares for good, in time they only shifted. Robin watched helplessly from thousands of miles away as her captain became more dangerous in the eyes of the government than she ever was. It sparked a question: What if…what if someday he did to her what she attempted to do to him? It was all too easy to picture, and for once Robin hated the morbid bent her imagination often took.
Luffy always offered her his hat in these new dreams—something he had never done in reality—before handing himself over to the World Government in exchange for her life. He would give her one last grin, offering up the assurance that this wasn't her fault. It was, after all, the duty of the captain to watch over his crew.
And Robin would always be powerless to stop him from succeeding where she had failed.
Invariably she would wake up in a cold sweat, heart pounding in her chest until she remembered that it wasn't real. They were separated for this very reason; when the Straw Hat Pirates reunited on Saboady they would be strong enough to challenge the world's monsters and win. Until then…
Robin settled back in bed, lying in perfect stillness. Even if she could go back to sleep, there was little point in willing returning to the terrors that haunted her mind. Better to stay awake and dream for the day when she was back to the one place she truly belonged.