Amnon falls in love with his step-sister, but in actuality his feeling is lust. He doesn't know what true love is. He has never seen it. There was no true love in his family. You recall Bathsheba was not loved by David. She was violated by David. She was not loved until a year or so after their child was born and died. Finally David comforted her after the loss of that child broke him, and only then did God begin to call Bathsheba his wife. It is probable that after he had lain with her he had all the disgust and self-loathing and guilt that goes with something like that. That is the model of love Amnon witnessed.
So Amnon lusts after his step-sister Tamar who is a beautiful girl. The law of God says if a half-sister and a half-brother marry, they were both to be slain. If a half-brother violated a half-sister, he should be slain. An incestuous relationship even as close as a half-brother and half-sister was totally illegal under the law of God. What does Amnon care. Big daddy not only committed adultery, which violated that law, but also murdered the woman's husband, which violated that law, also the 6th, 7th and 10th commandments and got away with it. Why should Amnon observe the law of God?
By constantly dwelling on his desire and feeding the flames of that desire and not being able to satisfy it, he becomes so frustrated he actually makes himself ill. He can't bring himself to violate her, not yet anyway, because she is a virgin and apparently acts like a virgin. She is a very chaste person. But Amnon has a friend. And as like seems to attract like, we get a little hint of Amnon's character here by the friends he has.