Why is my ex possessive over me?

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The situation is we go to the same B school and he never lets any other guy sit next to me. He would ask them to shift seats so we sit close to each other and always make a threatening face to other guys when they talk to me. And then sit unbothered. by the way he is an extrovert and I’m introvert so not much people I interact with. Also he never bullshit about me or say we are ex. But somehow always give me the angry dude face.
Why is my ex possessive over me?
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