Why can't I let it go?

I dated a guy from my job for 3 months, one month and a half was great, the other was hell, he used me and when he realized I was in love he didnt care and after that I was the one that said we shouldnt date anymore. after a month from that he had a new girlfriend ( which he was dating after we stopdating). I loved him but I know he wasn't the right onw for me. Yesterday ( which is like 2 months after we stop dating) I left a friend near his place at like 3am, and ped by his house to see if he was in. I just did it cos my friends house was like 2 blocks from his I dont normally do that, but I feel weird cos 2 months went by and I feel I still talk about him and wanna know what he is up to, cos we dont talk anymore. Is this normal? Should I be friends with him even tought he treated me bad, lied and lost respect for me? Seems to me he doesnt even care if Im ok.