Question

why poles of magnets always exist in pairs

Answer 1

No definitive explanation exists for why all magnets are dipolar.

There is no such restriction placed by the laws of physics that magnets should have two poles, unipolar magnets might exist. Scientist are currently working on finding if that is really the case. So, if they they do not find any unipolar magnets then we would probably need to search for an explanation.

One such should be that magnetic field lines form closed loops which originates at the north pole and moves towards the south pole and passes thought the material. A unipolar magnet would no form such closed loops so it would not exist in nature.