Yeah, I figured it would be more important for the viewer to feel something for her before the ending. But also, at the point in her life, she was 7-10 and had spent the first 7 years of her life locked up with only her mother, not knowing anything about the world. She would be shy. In my headcannon, it was Zack who brought her out of her shell, and imprinted his personality on her, making her more outgoing and forward. When she meets cloud in the church, she has the memory of zack asking her on a date, and tries to recreate it by being forward. But later, on the gondola, she admits she is a different person than she was when she was 15, and she wanted to feel what she did for Zack.
I really recommend everyone watch "Room". It kinda gives a bit more perspective on a child who grows up locked up