Question

why magnetic field lines forms closed curves

Answer 1

Magnetic field lines represent continuous curves that originate at the north pole of a magnet and end at the south pole. Inside the magnet they travel from the south pole to the north pole. ... So there is no source or a sink for magnetic field lines. The net fluxof the magnetic field through any closedsurface is zero.

Answer 2

Answer:

By convention. the magnetic field lines are the path  traced by north pole which emerges from the north pole  and goes to the south pole and inside the magnet the  direction of field lines are from south pole to north  pole, forming a continuous closed path.