Why does my friend act different in front of his friends?

We've known each other for 4 months and he's a genuinely kind and serious type of man. He's very warm with me in person, he initiates conversations and talks a lot to me. He has been there for me in my tough times and nobody has ever heard about it from him. However, he also has many times publicly supported me or been very sweet. I believe it has something to do with the fact that I'm few years younger than him and appear very delicate (appearance wise). However, whenever his closest friends are around he gets awkward. For example, today we were talking to each other (he initiated it) and his friends came. He got serious, mechanical and silent. They greeted me and I greeted them back. He was quiet but aware of the atmosphere though, there was a car far away and he told me to move closer to him. After they left he got normal, we talked for a whole hour. But I've noticed this before as well, last week we had a party where he got visibly awkward and startled when I talked to him in front of his friends although he tried to talk to me before when there was nobody else.

His friends don't seem to dislike me. But is he embarrassed to accept that we're friends in front of them?

Why does my friend act different in front of his friends?
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