Genetics/external fr. There is ample evidence that true asthma is genetically linked to the issues of allergy & eczema & frequency is higher when one or both parents are affected. Lower respiratory tract viruses often trigger both infectious wheezing & that of true asthma early on with infectious wheezing subsiding after 3-4y. Allergies often co-exist but less than 5% of events are allergy driven alone.
Allergies. A very common cause of asthma in children are allergies. One of the common blood components found in the mucus/sputum of asthmatics (especially children) is related to allergies (eosinophils). When we look at the "triggers" to asthma in children, they are often related to substances that cause allergies (ie animal dander, dust mites, etc).
Imbalance. Of the autonomic (automatic) nervous system. These nerves are controlled by the lower brain and mild compromise of oxidation in the lower brain initiates it. In my hard won experience it is related to poor diet, particularly sugar stuff because that is the easiest way to compromise oxidation. See my blog "oxygen, the spark of life" thatg you can google.