Why would a guy seem really keen, act cool and then keen again?

There’s a guy who I am really sweet on and most of the time he seems to be on me too, but I’ve noticed he’ll act keen and then the next minute it’s almost like he’s playing it cool and then out of the blue he’ll be flirty again.

What’s going on? by the way this is a work environment. when he’s keen, it’s almost like instinct but it’s very confusing,

Why would a guy seem really keen, act cool and then keen again?
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