Why North pole and South pole attracts?

 The end of the magnet from which the magnetic field lines always begin or "flow outwards" is called the North Pole (N-pole ) of the magnet, whereas the other end into which the magnetic field lines end or "flow inwards" is called the South Pole (S-pole ).

So, the two magnetic poles attract each other because the magnetic field lines want to attain the least energy, by shortening their length through the air. 

Magnetic field lines around a bar magnet: