Why magnetic field starts from north pole and ends in south pole? In magnets such as bar magnets the magnetic field lines are starting from North pole and ends in south pole..but I don't know what is the reason for it..why this happens.
 A: There's a logical sequence of definitions, from which the answer to your question follows...
The North Pole of a magnet is short for 'North-seeking pole'. It is the pole that settles down to point roughly geographically North when the magnet is suspended so that it can turn freely about a vertical axis, and is well away from sources of magnetic fields other than the Earth. 
The direction of a magnetic field at a point P due to any source, such as a magnet or a current-carrying wire, may now be defined as the direction in which the North Pole of a freely suspended magnet (basically a magnetic compass) points at P (having subtracted vectorially the effect of the Earth's field at P). [The magnet's or the wire's field at P may well be much stronger than the Earth's field, which served its purpose earlier in allowing us to name the poles of magnets, but may now be negligible compared with the field we're investigating.]
A magnetic field line is a line whose direction at any point along it is the direction of the magnetic field at that point.
We can now deduce that, outside a magnet, its magnet field lines will go from the North Pole of the magnet to the South. It's because (a) a field line tells us the direction of the field at each point along it, and (b) the direction of the field is the direction in which the North Pole of a compass points and (c) the North Pole of the compass will be repelled by the North Pole of the magnet and so points away from it! 
[Magnetic field lines are closed loops, so inside the magnet, they will run from South to North. This doesn't contradict what we've said before, which applies on the assumption of being external to sources of magnetic fields.]
[All this stuff about suspended magnets and compasses may seem outdated. There are other ways to determine which pole of a magnet is which and to determine the direction of a magnetic field. They are based on conventions (right hand rules and suchlike), but these conventions were chosen to be consistent with the old-fashioned definitions above.]
A: Because the magnetic north pole is in reality a south pole !
To be more precise : in a magnet, the north pole is where magnetic lines field exit. If you imagine the earth magnetic field as created by a giant magnet, the magnetic north of this magnet is at the south magnetic pole of the earth. 
A: Your question is like asking "why pi is the ratio of circumference to the diameter of the circle.
It is the very definition of pi. 
It has been experimentally concluded that magnetic field lines surround a magnet, which are closed figures. They just named it that way. They could've used cat or dog too, but that would be great since most of the namings have some underlying meaning and cat and dog is meaningless other than the fact that it represents the same thing as north and south pole.
A: In the question are some misunderstandings about magnetic fields.
In a permanent magnet the magnetic field is the result of the alignment of subatomic particles, mostly of electrons. In detail, subatomic particles have an intrinsic magnetic dipole and in permanent magnets these dipoles are aligned (frozen).
Any magnetic field is a closed loop. This holds for the magnetic field of the subatomic particles as well as for the macroscopic field in a permanent magnet, which is the superposition of the magnetic fields of the involved subatomic particles.  
In a permanent magnet the aligned particles are aligned somehow in chains. But the magnet is of a limited length and at the beginning and the end of the body the magnetic field leave the body and are able to continue even in vacuum. Between the ends of a magnet the magnetic field is bend and connects this ends.

Because magnetic fields in parts are going through the empty space to form closed loops it is possible to align other magnets. Having named the “poles” of a first magnet, the alignment of other magnets in the magnetic field of this first magnet allows to mark their poles. This was done many centuries ago because the earth has a magnetic field and some materials were found on the earth, which are permanent magnets. The compass was invented and the earth’s poles were named.

In magnets such as bar magnets the magnetic field lines are starting from North pole and ends in south pole.

So the field lines are not ending, they are loops. And some part of the loop is going through space. This is right for every magnet (if not closed in a donut). The magnetic fields of two magnets get in superposition thanks to the part of the field lines in empty space.
A: Because it is a fundamental property of nature. Magnetic field lines forms closed loops. Two bar magnets held over unlike poles (a north end and a south end) would attract. You can imagine a field line connecting the two ends. That is a loop by itself. 
In fact, all about magnetic field lines that physicists know is about as much as you know yourself, that the field lines trace out closed loops between ends of the bar magnet. 

(Field lines of a single bar magnet)
If you spread out iron fillings, they arrange themselves along this pattern of field lines. Magnetic fields are not really predictions of anything else. They were discovered in this fashion. That they always seem to form loops. 
I am aware that there are theories on the existence of a single pole existing all by itself (monopole). It still waits to be confirmed. 
