Why did he breathe heavily when we hugged?

He's a very shy person, he finally built up the confidence to talk to me face to face. I thought it would be awkward but it wasn't. i really enjoyed spending time with him. and when we were saying goodbye, we hugged. I hugged him tight and so did he, but he started breathing loudly then stopped right after we stopped hugging. what does that mean?
Why did he breathe heavily when we hugged?
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