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Birth is traumatic (for mother and baby). Even during a caesarean, babies tend to be pulled in awkward ways. Even withstanding this, if a child is already sensitive, the sudden and dramatic change in environment can be quite a stressful event.
Kids fall over all the time. Kids are resilient and their bodies adapt fast. A fall onto their butt may subluxate their lower back, pelvis or sacrum (tail bone). Because kids adapt so fast, they get up, move on and keep playing however the subluxation is still there just now their body and nervous system is growing to compensate for this subluxation.
To make learning easy. If the brain is getting constant static (or other annoying information) from a joint that is subluxated, this information can keep it quite busy. Instead of being able to concentrate on learning, this child may find it hard to concentrate or have to wiggle around all the time not realising it shouldn't be uncomfortable to sit still or not understanding why they can't take stuff in.
Because they grow fast. As a twig is bent, so grows the tree. Regularly when we take x-rays of adults, we are able to see signs of childhood injuries that were not dealt with optimally at the time and the body has grown to compensate for. The sooner such issues are corrected, the less effort the body has to put into compensating and the more it can put into perfection ;-)
Because life is stressful. Life in general today exposes us to a huge number of chemicals and toxins in the air we breathe, the food we eat, the chemicals we use to clean and bathe in. These toxins act as stressors in the body. Other stresses that can upset the nervous system of a child just as much as an adult include changes - new school, new teacher, ill family members, tension in family relationships, lack of exercise or too much exercise, too much sitting, too much artificial light at the wrong times just to name a few.