Take a picture of me!!!!!

So whenever I shoot events for something very specific I always have people that come up to me and ask to have their photo taken. After i've taken the photo the people walk away and never ask about the photos? 

I'm sure other photographers deal with this as well. The most recent was two nights ago at a college prom at the DCU center. I was there to take photos of my friend who was DJ'ing and concentrating on taking photos of his equipment and lights. While I was doing this people asked me several times for photos and then walking away without even asking to see the photo.