Traditional or "commanded" reasons for immersing in a mikveh have to do with marking life transitions, for example, from being not-Jewish to being a member of the Jewish people; from being single to being married.
Life contains many major changes, and with greater access to the ritual of immersion, people now use the mikveh to commemorate and sanctify some which have gone unnoted and unnamed by our tradition. Mayyim Hayyim has a growing library of ceremonies that address a wide variety of transitions, such as:
Fertility (
During the 9th month of pregnancy, after giving birth, upon weaning, following menopause)
Coming out as gay, lesbian, bisexual or transgender
Becoming a Grandparent
Becoming the Parent of a Bride/Groom/Bar or Bat Mitzvah
Embarking on/Returning from a Journey or Sabbatical
