[guayguay]

Why is it that an increase in the number of unpaired electrons in an element will result in an increase in magnetic susceptibility (magnetic attraction)? I know this happens but can't explain the science behind it.

[Corribus]

The (spin) angular momentum of an unpaired electron has an nonzero energetic interaction with a background magnetic field.  Because this is a potential energy field, a force is created between the source of the magnetic field and the magnetic dipole created by the (unpaired) spinning point charge.  This is analogous to the classic magnetic force between a moving point charge and an external field.