Soccer
(I have not read/listened to all the supplementary material regarding 01 & 02 yet so if any of my information here conflicts with a canon source I would love to know so feel free to inform me)
Sora started soccer because she was good at it. Before she ever thought to join a soccer club or learn the sport formally, she was already controlling the ball easily with her feet and constantly winning the playful games with her peers. She'd thought, being the big fish in the small pond she was, that she was the best there was.
After joining the soccer club she found that she was sourly mistaken. This actually only served to light a fire within her. She was an intelligent, athletic, and friendly child, she had rarely run into anything that hadn't been easy for her. Finding something that she had to put her all into to keep up with was one of the most amazing things she found in her life. She wasn't satisfied with just being good at something, she wanted to give it her everything.
She also learned that soccer was a game she would never truly get better at if she couldn't play nice with others. She'd never been bad at interacting with others but she'd never had to do the sort of teamwork where she had to truly depend on someone else and to likewise be depended on. She became engrossed with keeping one step ahead on the field, using teamwork to as an advantage, pushing every imaginable limit physically and intellectually until her brain became overly focused on soccer and nothing else.
Of course, her grades suffered for it, she wasn't a genius or anything and putting study-time aside in favor of extra practice time can't really improve someones grades. She'd often come home covered in mud, scrapes, and bruises and loved every moment of it.
Soccer actually became her primary motive for wearing her hair short, whenever it started to grow out past a certain point it was always in her way on the field. She found a length that she was most comfortable with (and found really cute) and kept with it.
The more she pushed herself with soccer the happier she felt. That anyone would deny her playing or force her to betray her teammates, her friends, was so unthinkable to her that she could only interpret it as hatred.