Classic!
"I don't get anything. I'm on diet! I'll just take a bit of what you're eating/drinking!".
Apparently women feel less guilty about breaking their diet if they are cosuming a portion that doesn't belong to them.
Plus there's the envy factor: Anything looks more desiderable if other people are enjoying it. And that is how advwrtisement look.
And finally there's the love factor: Sharing anything that goes to mouth with your beloved one is more romantic than consuming something on your own.