why do magnetic field lines leave the north pole and enter the south pole? suppose we have a bar magnet

|S_______________N|
...
we can imagine it to be

|S-N-S-N-S-N-S-N|
So, why do we see the lines leave the north pole and enter the south pole externally?  What happens internally?
 A: Magnetic field lines do not go anywhere. Field lines are useful for visualizing vector fields. These are not physical that are actually present at certain locations. And the direction you are talking about is a convention. By convention, the field lines are taken to direct away from the N-pole and towards S-pole. Internally, these field lines complete a loop :-

Note: Reading this will help in understanding the nature of these field lines better link
A: First at all the magnetic lines 'connect' the two poles. There is no flow of energy or material nor preferred direction of these lines. In this sense the north and the south pole are equal.
Then, the magnet field lines are not connecting only the ends of a magnet bar (see this picture).
Internally exist small dipols, which are orientated more or less. This can be from natural materials or more powerfull this are pressed from powder magnets which get there orientation by the help of a magnetic fields during the production process.
Magnetic monopoles are not existing, hence there is always a "chain" of material dipols inside the magnet bar and outside there is a "chain" of electromagnetic dipols connecting the two poles together.
