I sometimes have a "gas" issue. And frankly, I get all embarrased and it cramps my style if someone is drafting. Every now and I get I can't help it and the drafting quickly ends.
For this insane reason, as well as other things that have been mentioned -etiquette, subtly forcing the leader rider to speed up and ride differnently, trust, etc. etc. Wheel sucking isn't cool.
What's that smell?