Haruhi can't reciprocate her feelings for Kyon, because she's in love with John Smith, who is Kyon, from the past, because he travelled through time.
Because of this, she is in denial that she loves Kyon too. Even though John Smith is Kyon. And Kyon would reveal to her that she's god (god becoming self aware is bad, apparently) if he told her he travelled through time and shit. Even though that's what she wants most. I think. Okay.
Thus, tsundere bitch.
Holy shit this shit sounds like bad fanfiction when you write it out obviously like this.