Guys, why did he take a deep breath after walking past me?

He has a crush on me, and I have one on him but we just won't tell one another. Anyway I watched him after we passed one another to see what he'd do and he took a big deep breath with a straight face. And nooo it's not because he thought I was beautiful, he avoided eye contact with me as soon as I looked towards him. His faced looked really sad today too.
Guys, why did he take a deep breath after walking past me?
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