What was he thinking by staring?

My friends and i got a ride from our older guy friend. I was sitting in the front and i turned around to talk to my friends. I noticed he was stared at my boobs for a second or two. it was a quick glimpse. what does this mean? i wasn't even showing cleavage. I think it was his first time but i think he checked me out at the restaurant too...so what does it mean? he kept talking to me and asking me about what types of guys i like and stuff....help?