I am assuming you are talking about magnetic poles here. The reason behind this is magnetic flux lines, flux is responsible for force (correct me if I am wrong). So, flux lines start from north pole and end on south pole. read more
The same poles repel each other because they are both negatively or positively charged. The same poles cannot exchange electrons and become neutral. This is the same reason why magnets attract neutral objects like iron or nickel towards them. read more