Why do Ex's always come back?

My ex and I were on and off last year. I noticed that he always came back when I was healed and happy, going out and having fun. This time, he said he doesn't see a future with me cause I have a bad personality and I am negative all the time. Which is not true, he didn't see how he treated me to know why I reacted that way. This time I want him gone, I don't want him popping up out of the blue asking me to take him back. I want nothing to do with him at all.  I blocked him on everything. Why does he do this all the time?