The human brain is supposed to develop in a certain pattern. Unfortunately, this isn't always the case. A step may be skipped, or damage may be done that removes some of the necessary develpment.
When this happens, we see an imbalance in development and a failure of higher level functions to appear on one side in relationship to the other. Think of this like a house - if you don't have a good foundation, you can only develop the house to a limited extent before it comes crashing down.
In contrast, if you have a strong foundation, you can build a much more complete structure on top of it.
Another example would be compound interest. If you miss just one or two cycles of compounding interest, your end total is a fraction of what it otherwise would have been (in this case in one side of the brain compared to the other). These missed milestones or steps back have a major impact not only on typical brain functions, but also on intelligence and behavior - leading to disorders like ADHD, autism, autoimmunity, dyslexia and more.
Any setbacks like this need to be properly rehabilitated so that we can achieve - or regain - optimal function. The level of repair of these areas will determine the levels of health or disease that can be enjoyed.
Fortunately, most of these issues can be balanced out with proper treatment allowing more ideal brain function - and thus less disease, higher intelligence and better behavior.