Why would a guy change tone down flirting behaviour with a girl after she said she liked him?

If there’s a noticeable behaviour from a guy flirting with you eg prolonged eye gazing, checking you out, getting nervous and a sudden change after you tell him you like him, still super friendly, but without the intense flirting, more like relaxed, what does that mean?

How can he change his behaviour just like that?

Why would a guy change tone down flirting behaviour with a girl after she said she liked him?
Post Opinion