It is not clear why some people develop asthma and others don’t. It could be as a result of both environmental factors and family history.
Asthma can be triggered by a person being exposed to substances with cause allergies. These substances (triggers) may affect certain individuals and not others.
Asthma triggers may also include physical activity, preservatives added to food, weather conditions, other health conditions, or certain medications.
Have a read further on Allergy and Asthma and Risk Factors for Asthma