People generally do not enjoy the smell of their own farts. However, they will tolerate the smell because it is something that came from them, so it cannot be that bad. Compare that to licking sauce off your fingers rather than the fingers of a stranger. Other people's hands are considered dirty just as other people's farts are more disgusting.

In addition, people are generally embarassed of their own farts and would not want someone else to know they farted. The two best giveaways are the sound and smell. Perhaps they can quiet the sound or let one slide when no one is around. However, the smell may linger for a while. The last thing a person wants is to have someone walk up and smell a dirty fart. It almost seems to the other person that they were just sitting around farting and enjoying the smell of their own farts. The truth is that they were hoping to rid the air of the odor before someone approached the area.