Sakura had always thought that Kakashi didn't notice her. She was never strong enough, never pretty enough, and she would never be old enough. Everything had changed mysteriously, however, after a simple conversation she'd had with the Hokage. (Long kakasaku oneshot)
Sakura Haruno was abused and abandoned at a young age. Her father held only resentment towards her ever since her mother died.
Because of his hatred towards her at such a young age, she feared trust. But all of that changed when she met Kakashi.
He helped her when they were younger and he helped her again when they were older. He helps her with more than just trust, though. He helps her understand what it is to be loved and love. And for that she is grateful to him.