Why magnetic field lines always starts from north to south?
Magnetic field lines depict the direction of magnetic field at any given point. It is easy to draw these field lines, by using a magnet and a compass. If we place a bar magnet on a piece of paper and put a compass near one end of it and start moving it, we can trace a magnetic field line. We commonly use a convention that these lines emanate from the north pole of the magnet and complete a loop and enter the south pole.
Thus, it is only a convention by which we decided to use a direction for these lines, just to ensure that everybody is using the same directions. The actual direction may be different. For example, we knew about the north and south poles of Earth and we named the end of magnet which was attracted to the north pole of Earth as the north pole. South pole of the magnet was similarly named. However, it was later found that Earth's north pole is close to its magnetic south pole (and that is why magnet's north pole is attracted to it).
Thus, it is purely a matter of convention which gives us the direction of magnetic field lines.
Hope this helps.