To the OP:
Genetics. Everything is always genetics.
All human DNA is the same, but there are triggers.
DNA strands are triggered/activated by what we do, eat, radiation, chaos patterns, what our forefathers did... Hugely complicated.
To us humans, it is indeed a lottery of sorts.
The root, though, is genetics as we are organic beings. Our minds are not ethereal.
Genetics is where being a human STARTS, but then things can affect how you are even before you leave the womb. Does having a mother that has an odd dieting habit affect your genetics while you were in the womb?
Brain cells do not regenerate quickly, thus are not as affected by radiation, but the interconnection of neurons in the brain can change, affecting thought.
Also, I think Victor and Ajblaise have good points.
If a person has higher cognitive activity, wouldn't it feel good for them to use it? It can also make sense because if we are around external things, our minds overreact to it, spending more energy than is needed, making things disorganized. When we spend time alone, our mind can clear the clutter.
But if it lasts too long, a person gets 'stuck inside their head' and their internal ideas truly affect reality, instead of just coloring it. We get lost in the vaults.
And to clarify: Introversion is when a person gains energy by being alone, rather than by doing things with other people. No person can be 100% I or 100% E, there is always a ratio between interacting and meditating (meditating, in this sense, is focusing thoughts inwards or doing solitary things). Ns tend to be more focused on the Meditating on subjects and Ss tend to be more focused on interacting.
My train of thought is unraveling, I hope the post makes sense.