There isn’t a clear-cut answer as to why some people have them while others don’t but scientists do have one particular theory. The hygiene hypothesis is the idea that excessively sterile environments are contributing to the development of allergies, asthma, and other autoimmune illnesses. Kids in wealthy countries lack exposure to allergens and other germs that aid in developing a healthy and functioning immune system. While the hygiene hypothesis isn’t the definitive answer to seasonal allergies, it is a starting point.