Your brain actually stops picking up on lots of scents. If it didn't, you'd be overwhelmed by the constant sensory input of your own smells (and the ones that tend to surround you). It's the same reason a stinky room will get more tolerable after a few minutes of being in it, and why wearing one perfume for too long can make you think that it doesn't smell as strong.
So it's not surprising that your one-of-a-kind farts seem innocuous most of the time. But there's also science behind your sense that other people's gas smells nasty.
In modern, underwear-clad times, flatulence isn't usually a health hazard. But when you think about where that gas is coming from and what kinds of unfamiliar microbes it might carry, it's not surprising that we've evolved to find it disgusting.
But everyone's different, and maybe you're totally nonplussed by others' gaseous emissions. Just like all forms of disgust, this reaction to scent varies person by person. In fact, some research suggests that people who are more anxious or conservative are more sensitive to stink. Conservatives are more likely to react to disgusting images as well, and scientists believe this reaction has to do with a distrust of the "other" -- an instinct to protect one's own body and community.