Why am I always turned on?

I'm a 15 year old male. I'm always turned on. I get these moments when I'm with my girlfriend to were I just want to like do something sexual. It was never like this. But it starting to get a little weird. I get these 'woodys' when I'm with ny girlfriend and she has seen it before. She just laughs. Ive been talking to my girlfriend about sex. And she wants too but she also wants to wait. And her playing hard to get seems to make me turned on even more. What's the reasoning for this. Haha