Authentic alienness
Monster romance's non-human love interests are most compelling when their alienness is authentic and consequential: when their non-human nature generates specific obstacles, specific misunderstandings, and specific forms of connection that would not be possible in a human-human romance. The vampire who experiences human warmth as a reminder of what he lost rather than simple pleasure; the werewolf whose pack bonds create loyalty obligations that conflict with individual romantic attachment; the demon whose understanding of love is filtered through a culture where the concept does not translate cleanly — these specific aliennesses create specific dramatic possibilities. Writing authentic alienness requires imagining the non-human perspective from the inside rather than simply applying strange traits to a fundamentally human character.