Guy's Behavior

Guys, Why does this guy look at me sad?

So there's this guy that i USED to like and he has a twin brother. His twin brother and i have never talked but once like a year ago. A few months ago i started to just randomly notice, everytime we saw each other in the halls at school, he looked at me sad and depressed looking. Like he'd just look ag me and when i looked back he'd look down and away looking really upset. When i walk past him he'll just clench his jaw and look down and away from me.
I've never did anything to make him upset at me though.
Another time i was standing in the hallway talking to friends and it happened to be outside his classroom that he was in (this is in class change). And in the corner of my eye, when my friend yelled my name, his head shot up and stared at me. And when i looked back, he looked like a sad puppy that i just kicked, legit. And we made eye contact when i looked back 3 more times.
in attempt that i was gonna talk to him, i added him on Snapchat but he wouldn't add me back... I know that doesn't mean much but still.

So im confused on why he's all sad everytime i come around. When he's in a GROUP of friends he looks normal but glances at me a couple times. And just to note, he does NOT have a naturally looking sad face either.

So whats your opinion on why he's like this around me? And what should i do? Because i have no idea. Thanks!
Guys, Why does this guy look at me sad?
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