Why is he excited to be with me when we are alone for like a few mins at work?

Guy friend at work only is excited and more interactive when we are alone at work or there's like 1 or 2 people who aren't trying to talk to us...

But if there's multiple people especially other girls who flirt with him, he doesn't show me attention. I don't know why.

But once everyone leaves, he perks up to me and we feel that spark like old times.
Why is he excited to be with me when we are alone for like a few mins at work?
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