sick with frustration: The story shows that Amnon's "love" for Tamar (2 Samuel 13:4) was not healthy love. It was an unhealthy sexual obsession that made him emotionally distressed.
She was a virgin: Amnon’s fixation grew stronger because Tamar had not had sexual relations.
it seemed implausible for him to do anything to her: The law forbade marriage between a brother and a sister (Leviticus 18:9, 11; 20:17; Deuteronomy 27:22). However, Amnon did not seek marriage with Tamar. He wanted only to have sex with her.