Why are some alleles dominant and some recessive? explain with an example?

It is determined by the instructions given by the genetic material and the gene that wether an allele would be dominant or recessive. A recessive allele is believe to be non functional as it does not express in presence of the dominant allele. 

Example: In blood groups, A and B are dominant while i is recessive. If 'i' is present with either A or B, the blood group would be A or B respectively.