Lyric discussion by hayleyfacee 

The guy likes a girl and he's pretty sure she likes him too. They act like friends but they both sense that they want more from each other. But for whatever reasons, they are both scared to fully admit it or pursue anything. The guy notices that they are behaving like animals in the sense that the girl plays hard to get, and the boy must chase her down (if you notice, in the wild, the girl animal usually blows off the male a few times before she finally gives in). Even though they act like they aren't that incredibly interested, he knows they both are, hence the way he mentions that they're playing pretend.

He's scared he "won't get out alive" or "I won't sleep tonight" because it's driving him crazy that they're not doing anything about their attraction to each other.

The whole chorus seems to be the guy saying "hey, I want more than what we currently have. I want to pursue this further. Go ahead and tell me how you feel, make a move, take a bite of my heart, let me know you want me too so I can give into you fully. What are you waiting for? Things are never going to change if we keep pretending. When he talks about the chemicals kicking in, I think he's talking about he feels his feelings for her hitting him harder and harder as time goes on, and it makes him nervous and scared when she's around, so he wants to run and hide because he likes her more and more as time goes on, and he's scared of getting hurt so he's scared of liking her more. "I do it everytime, you're killing me now" This means perhaps that everytime he has a chance to pursue things further he chickens out, and it's killin' him that she makes him nervous.
"I won't be denied by you, the animal inside of you" the guy and the girl act so normal around each other, so calm and such, but he knows that somewhere inside of her, she wants him and she's sexual, and he just wants them both to be able to unleash how they really feel AND indulge in physical relations with one another, and his animal instincts are kicking in because, like in the wild, the male animal takes on the attitude that he will NOT be denied by the female animal.

Finally Hush, hush The world is quiet Hush, hush We both can't fight it It's us that made this mess Why can't you understand? Whoa, I won't sleep tonight

means that the world is quiet when they are around each other because they are only focusing on each other. He senses that they both can't fight the undeniable attraction to one another. And the mess he's talking about is the mess that they created by flirting and carrying on for a long time without purusuing their feelings, thus creating confusion and an animal-like game out of things, and he won't be able to sleep at night because things have finally gotten to the point where he can't stand it anymore and wants to pursue things further with her.

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