why the magnetic field lines always begin from N-pole of a magnet and end on S-pole of the magnet

Magnetic field lines are closed lines. And like any closed lines they don’t have any beginning or end point. The direction of magnetic field line is taken to be from North Pole to South Pole outside a magnet. Within the magnet the field lines are from south to north. The direction from north to south is a convention based on the alignment of a magnetic compass.

The sense of direction of magnetic field lines is a convention that has been followed since the discovery of magnet and its properties.