Q: What is the reason behind the bad smell of durian fruit?

ArabiaWeather - In Singapore, it is forbidden to carry durian fruit on the subway, and even in some hotels in the city, because of the fetid smell of that fruit.

The reason behind the bad smell of durian fruit is that it produces abundant sulfur compounds with a stinky smell, and sulfur is the compound that causes the unpleasant smell of eggs or bad breath upon waking up from sleep.