Lagers simply take more time, need more yeast cells, and require colder temperatures than ales. And, for at least a day or two, they can also smell far more horrendous.
Sulfur, or more accurately hydrogen sulfide, is a gas that features the unmistakable aroma of rotten eggs or of geothermal features. My first homebrewed lager gave off so much sulfur during primary fermentation that the garage smelled like a Yellowstone mudpot for close to a week. The Helles turned out great, but it was certainly an exercise in trust. Even when you visit a commercial brewery, you’ll often pick up a not-so-subtle whiff of eggs gone bad. It’s just part of the experience.
All yeast strains produce some amount of hydrogen sulfide during fermentation as a by-product of sulfate processing. According to Angela Linderholm and Linda Bisson of the Department of Viticulture and Enology at the University of California, Davis, Saccharomyces cerevisiae can produce up to 290 micrograms per liter of hydrogen sulfide in wine. The human threshold for detection is a very low 0.01 micrograms per liter.