I really like writing this story.
Sirius Black prided himself on being a lothario. He was not unaware of his effect on the opposite sex. Often, he took it for granted. Sirius had a way with girls—he'd flash a smile or tousle his hair and the girls would swoon.
Not Myrtle. She was different. He couldn't figure her out, even after all the time they spent together. Myrtle was not transparent to him.
Myrtle was a private person; she didn't share things about her family. She didn't share the torment of her father's abandonment or the pain of her mother's neglect with anyone—especially Sirius.
Despite all of the time she spent with the boy, she did not trust him. Myrtle was aware of the particular charms of Sirius Black. The very thing he prided himself on was what repelled her. She feared he would be like her father; she feared he would treat her the same way. Myrtle could not bring herself to trust Sirius.
When Sirius finally gathered the courage to ask Myrtle out, he did so in front of the entire Great Hall. He was sure she felt the same. However, he was met with embarrassment as she said no and fled.