Basically, the cause of left or right handedness is genetics. Gene of a human being happens to be inheritable traits that are transfered from parents to an offsprings. From simple sense of reasoning, it can be said that an individual that is either left or handedness handed got it from his/her parents, most times from the father since he is usually the one with the dominant genes.
Apart from genetics being the primary cause of left or right handedness, there are other factors that can also contribute to left or right handedness such as societal beliefs. Some trandition have the belief that it is wrong for someone to be left handed. Due to this erroneous belief, someone that is left handed could be compelled to be right handed.
But for my case, I have been a chronic left handed someone right from the day I was born. And this is attributed to inheritable cause of left handedness because my dad is also left handed.