Why does my ex boyfriend keep staring at me from afar?

So, I broke up with my ex about 4 months ago. I tried to mend things with him but it didn't really work. We still talked almost every day though until we got into this really big fight about a week ago. I called/told him some pretty horrible stuff and he got his brother to gain up on me and the brother told me that my ex "doesn't want me" and then my ex blocked me (rightfully so) on all social media. School started recently and my ex is in one of my classes. I've noticed that he looks at me occasionally in my class and when I pass him in the hallways. I don't want him back because he's a shitty person but I'm just really curious as to why he keeps doing this. It's kind of weird.
Why does my ex boyfriend keep staring at me from afar?
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