Horrible smells tend to be made up of more volatile molecules These are the molecules that tend to break down from their source and hang around longer - so there's usually more of them in the air to be smelt than "pleasant" smells. This is often why they are perceived as horrible - because these volatile molecules are also the ones that are most reactive (i.e. the ones that can actually do our body physical harm) - so we perceive them as horrible because they pose an actual risk to our health.
We also react more to these smells (because they potentially pose a risk to us) so we're more likely to actually notice them and continue to notice them.