Dana was surprised at how strongly Grand reacts towards Bobby. They are best friends. She doesn't see anything wrong with being friends with Bobby; also they are not in a relationship. What he said from before wasn't considered as a love confession, right? Is what Dana is thinking, confused with the sudden anger from Grand.
"I admit that it was my mistake forgetting the time and being late," she said quietly. "If it's what makes you angry, I'm sorry."
Grand seeing Dana being submissive and easily admitting fault and asking for an apology, brings him too much shame. Dana's isn't in the wrong. A few minutes late is not something that she should ask sorry for. His attitude is the one that is questionable. If there is someone that should be asking for an apology it should be him. Yet, Grand couldn't get the words out of his mouth, and he's not going to deny that it gives him unexpected pride that Dana was the one who said sorry first.
