Why is my ex smiling and staring at me?

After my ex and I broke up we agreed to stay out of each other's life, but the next time I saw him he was staring and grinning at me... I totally ignored him though. So today a mutual friend of ours, kept telling me that my ex told him to tell me he said hi, but I didn't respond and would change the subject. I would like to know what's the deal? Why is he smiling at me and telling me hi after our agreement?
Why is my ex smiling and staring at me?
Post Opinion