why righthand drive in thailand

Why a country has left-hand drive or right-hand drive lies in history.
First the definition: left-hand traffic is driving on the left side of the road. The driving is sitting on the right side of the car and is shifting gears with the left hand. That is a right hand drive.
And in right-hand traffic you drive on the right side of the road and are sitting on the left side of the car. That is a left side drive.
Why a country drives on the left side or the right side of the road lies in history.