Why was he last night acting like that?

Last night I (f, 18) ate a fruit alone on the balcony when the mother of my current host family sat down next to me and started a conversation with me. She seems to like me. She listened to me carefully and wanted to know everything about me. The son (19) then came into the living room and saw us both talking together and smirked up to both ears ike crazy. Does anyone know why?
Why was he last night acting like that?
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