Why do I still think about him?

I broke it off about 4 months ago. The relationship itself lasted 3 months and it was my first serious relationship. I was always annoyed by him and I can't remember why I was so unhappy. I have a new boyfriend who I really like. But I find myself reminiscing about my ex and our dates and the memories. I brought him into my life. He met my family and we did so much together. I know my ex still likes me. He was crying 2 weeks ago because he saw me and he couldn't handle it. Today he was pissed that I was showing off the flowers my boyfriend got me. He got mad that I never did that stuff with him. Then he requested I unfollow him on all social media so I did. I jist cant figure out why I think about the unhappy relationship we had.