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

Okay me & this guy went out for 1 month,we broke up because he said we were moving too fast. We have been fwb since then because I wanted to do that (because I was sad without him).But ever since then me & him have been friends with benefits.I lied to him a few times too which I feel really bad for still. But he told me we couldn't be anything serious. Because he doesn't trust me & no he is not using me we both get something out of this. But the thing is when he calls me or texts me I get this happy feeling.