Sometimes he seems interested and other times not so much?

It catches me by surprise, but sometimes a guy I know will be in a very flirty mood and look in my eyes and tease me and flirt with me.

But then sometimes he'll act just like a friend, not really showing any interest, and when he's like that and I try to tease him or be flirty he seems a little indifferent, kind of like he's telling me "what are you doing?"

is he interested or is he just flirting for fun when he feels like it?
Sometimes he seems interested and other times not so much?
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