Why does he act interested one minute and distant the next?

Lets say there's a guy who told me he liked me, was very thoughtful, and took lots of time to get to know me better and just hang out. That went on for half a year, and now it seems like his enthusiasm is sort of dampened. We're not official yet, but just last week he was doing things that made it almost for sure seem as though he still liked me.

Why is he doing that? Acts very much interested in me and flirty for weeks, then a few days here and there he kind of withdraws?

(He definitely doesn't have another girl btw.)
Why does he act interested one minute and distant the next?
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