(MAJOR SPOILERS) Harley was in love with...

... his sister.

There are some hints that there was an Oedipus complex type of situation going on with his mother. But the person he really was in love with was his sister, Amber. And his sister was also in love with him.

Now, we are NOT supposed to read this situation as being incest. Harley and Amber were brother and sister in a metaphorical sense. If you were to look at them side-by-side, they do not resemble each other. Ex: Amber's hair is blond, her lips are bigger than Alex's...

When Harley confessed to his shrink that Amber had "touched" his penis when they were younger, I think he meant that she "touched" him metaphorically - as in, he stirred desire in him. Amber wanted to be desired by her brother. That's why she roamed around scantily-clad. She wanted to make him desire her.

When Harley saw Amber on the couch, having sex with a random guy, he burned the couch because he was jealous. Amber wanted to draw attention to herself. She wanted Harley to feel jealous when seeing her with other men. She wanted his attention.

When Amber found out that Harley had intercourse with the neighbour, she became insanely jealous. And she left with some random guy. She was placated by Harley eventually... and she came home. Only to take her anger out on her neighbour, by killing her.

There was so much sexual tension between Harley and Amber. And I personally thought that the actors who played those roles looked really good together. Harley was ultimately too timid to makes any moves on her. This is owing to his rough past and low self-esteem, and possible abuse from his father. Basically, Harley didn't think he was good enough for Amber.

BUT, Harley still made the ultimate sacrifice for Amber - he went to jail for her. No man would do that for a woman, unless he deeply loved and cared for her.

It's also important to note that in spite of Amber's behaviour, she loved one person only: Harley. For her, giving her body to other men didn't mean anything to her. They were just ploys to get under Harley's skin. But when Amber sensed that Harley had become emotionally attached to another woman, that truly broke her heart, and she felt rejected - like the shrink said.

Amber loved Harley when they were young kids. She sought comfort from him, because she was afraid of her father. Sleeping with Harley in Harley's bed made her feel wanted and desired. She had a thing for him even back then. My guess is, that over time, she got frustrated with him for not responding to her the way she wanted, and that's why she kept chiding him. She wanted him to be a man: HER man.

Theirs was a twisted love story - but a love story nonetheless.