Why does he stare at me angrily?

there's this guy that just stares at me. We'll make eye contact and stuff but im usually the one to look away. Sometimes, he looks sad or just a blank face when he stares. He'll stare at me when he's talking to someone and im walking by, he'll like look at them and back at me repeatedly.
I've noticed tho that if look in his direction or just look at him and he's not staring, he looks angry all of a sudden. But there's no reason to be mad at me bc I've never done anything to him and we never even talked! Like ill look at him and he'll just look down and look angry or upset. And I've paid attention to him in a normal situation and its not that his face looks sad or mad, his face changes towards me!
He seems to be a confident guy that talks to a girl he likes and stuff and makes it obvious he likes them.
I don't know if this makes a difference, but i USED to like his brother, and him and his brother probably still think i do for some unknown reason. I have no contact with him or his brother by the way. I was never really close at all to his brother anyway.
Why could this guy be acting this way? I know staring isn't EVERYTHING but its consistent and he only really stares at the people he's talking to.
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by the way this has been going on a few months and even some about a year ago (idk if he stopped or i just stopped noticing for a few months bc i just restarted noticing about 4 months ago)
Why does he stare at me angrily?
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