According to hund's first rule, a set of degenerate orbitals are singly occupied first, before the second slot in any of the orbitals are populated. this is quite intuitive because electron-electron repulsions would make an atom more unstable if the electrons start filling two at a time in a single orbital. however, the rule also states that in the ground state, electrons that singly fill the orbitals will adopt the same spin. why is it necessary for the electrons to have the same spin?