My personal stance (or headcanon, if you'd prefer) on this would be that Fate, from the point she was born to where her mother died, relied on others to tell her what she liked and didn't like. Her mother would point at something and say something along the lines of "Remember? You love this."
If Fate answered with an honest "no," she'd get beaten because she was "a bad daughter" for not being more like Alicia. She only had her sister's memories, not her mannerisms, and that greatly frustrated her mother. So Fate trained herself to always agree--until the day she finally disagreed and stood up for herself and her own rights as a human being.
But when Fate entered Nanoha's school, she was a still bit uncertain about her own opinions and latched onto Nanoha for her own viewpoint. Fate didn't want to disappoint anyone she actually cared about for being "the one with a different opinion," so she always innocently agreed with Nanoha...because that planet was, to her, "Nanoha's world," not "Earth."
I digress.