Why do magnets have north and south poles instead of a single pole?

Magnetism arises from electrons, especially their spin and motion around atoms. In most common magnets, many atomic magnetic moments align in the same direction, creating a field with two ends. Those ends are called north and south poles, but they are not separate substances; they are opposite parts of one dipole field. No isolated magnetic monopole has ever been observed in nature. This dipole structure is why breaking a bar magnet does not produce a north-only piece and a south-only piece.