Why are there some right-handed people and some left-handed people?

There are many things in our body, one on each side.
Hands, feet, eyes, etc. In fact, the brain inside the head also has a right brain and a left brain.
The brain is an important part that gives various commands to the whole body to move and rest.
The right brain controls the left half of the body, and the left brain controls the right half of the body. This has a lot to do with right-handed and left-handed people.
It is believed that the left and right brains have different roles.
One is to think logically, such as speaking words, writing sentences, and calculating. The other is to take the image and think sensuously, such as looking at pictures and figures and listening to music.
For many people, the left brain is responsible for thinking logically.
Therefore, there are many right-handed people.
Some left-handed people understands words with their right brain. 
As mentioned, it is said that there is a deep relationship between the position of the part of the brain that understands words and the dominant hand.
It is also said that the dominant hand is determined by which brain works actively.