Robin wasn't supposed to want to live. She was supposed to just die with the rest of her village, and yet she was a survivor... the only survivor.
The World Government was after her, had been for years, and she should have given up and stop trying to fight back, to run.
She was the sole possessor of a forbidden knowledge: she was not supposed to be there any longer. And yet her friends, the crew she formed a part of, defended her, stood by her side.
It was because of them, through them, that she discovered she wanted to live. That she deserved to be alive.
Luffy had asked her if she wanted to join them, if she wanted to be alive, back when he declared war against all 170 participating nations! Madness, but he was always a bit mad, wasn't he?
So many thoughts had flooded her mind, and in that moment, she realized she was more than just a devil child, that she deserved a chance to find her calling, to wish for a future.
Now she traveled with them, as childish as they might be. And even if she'd never admit it out loud... she was quite fond of that merry bunch of lunatics.