why some people are borned only left handed and not right handed?

@ rutika , 
Its till now a mystry . 
in 2007 , scientist discovered a gene which has something to do with the left handedness , but till now the researches are going on , 
and as of now , the scientists are saying that it have something to do with the genes , the gene of left handedness are inherited from the parents , but its not yet sure as of now . 
AND ACC TO SURVEY ONLY 10 PERCENT PEOPLE OF THIS WORLD ARE LEFT HANDED . 
HOPE IT CLEARS YOUR DOUBT 

ALL THE BEST ..