The bacteria living inside animal and human intestines is responsible for eating up the fiber and hard-to-digest parts of grains and vegetables in our food. When they do this, they give off vital nutrients we cannot get through food alone. For example, humans have no reliable source of Vitamin K -- so we have E. Coli bacteria in our lower gut that produces it for us. There are somewhere around 500 different of types of these in the healthy human gut. We are born with few or none and acquire them over time and exposure.
When the bacteria eat up the cell walls of grains and fruits/veggies and convert it into vitamins, they also give off nasty smelling gasses containing sulfur. Some people have more of one species of bacteria and less of others, which can make them extra sensitive to certain foods such as onions, garlic, bran, or beans. This is why two people can eat a bowl of chili and one of them gets no gas while the other turns into a ticking time bomb of stench.
Some individuals are also lactose sensitive or intolerant, meaning a meal with any milk, cream, or cheese causes their bowel to move food through too quickly. This combines the normal digestive gasses with undigested food -- meaning it can smell like bad gas plus icky food smells.
An offensively gassy person is someone who ate the wrong food for their personal gut. A persistently gassy person is someone who is eating the wrong foods often.
Posted 1503 day ago