Why are magnetic field lines closed curves?

(N/A) By convention,magnetic field lines are considered to emerge from the $North$ $Pole$ and merge into the $South$ $Pole$ outside the magnet.
Inside the magnet,the direction of the field lines is from the $South$ $Pole$ to the $North$ $Pole$.
Since the field lines form a continuous path that starts and ends at the same point through the interior and exterior of the magnet,they are considered closed curves.