It's because Spike was her only friend, the only closest friend she ever had in her school, everyone else in Crystal Prep were mean to her, she was shy, alone, awkward, anti social, depressed, being black mailed by her principal etc, that's why Spike was able to get through to her and no one else, she cares deeply for him and him for her, they have a strong bond, cuz Sunset was just someone Sci Twi just met and barely knew her, and didn't help the fact that Sunset yelled at her and made her cry. :3