I think we train one hand, then use it cuz we use it all the time, and by the time we wanna use the other, then its stupid, cuz u never taught it and dont have the time
When you're born you can use both hands normally it's until later on, you use the most times of which side you used.
Like if you fell too much on the right you have a great chance of being right-handed while the same example can be said to being left-handed. Sometimes it's due to seeing your peers or getting forced. It all depends when your around 0-3 years old and it will be almost permanent at 4 years old