Why does a magnetic dipole experience torque in an external magnetic field?

A magnetic dipole experiences torque in an external magnetic field because the field exerts forces on the dipole that tend to rotate it into alignment with the field lines.12

When a magnetic dipole is placed in a uniform magnetic field, it is subjected to a torque $\tau$ defined by the equation:

$$\tau =mB\sin \theta$$

In this expression, $m$ represents the magnetic moment of the dipole, $B$ is the strength of the external magnetic field, and $\theta$ is the angle between the dipole moment vector and the magnetic field vector.12

The torque acts to minimize the potential energy of the system by rotating the dipole until its magnetic moment is aligned with the direction of the external field ($\theta =0^{\circ }$). The torque reaches its maximum magnitude when the dipole is oriented at a right angle (${90}^{\circ }$) to the field and becomes zero once the dipole is fully aligned.12