Why can't I get you out of my head?

I swear .... I can't stop thinking about him! I will just find myself thinking of him all through out the day! I don't understand because clearly he has no interest in me and I should not have any interest in him but, I just can't get him out of my head! I hear a song and think about him .... I just start drifing into a daydream and remember the nights we talked and how he made me feel and keep replaying our last conversatin together! I just looked through his pictures on FB ( I can still do that even though he blocked me) and I am trying so hard to remember his voice but I am forgetting the way it sounds and I start to think was it really him? Why is he so freaking specail? Why am I still obessing over him! Please just get out of my head ... thanks!