Beethoven never married, but was often romantically attracted to unattainable (and often married) women well above his social class.
After his death, a long letter was found amongst his personal effects in his handwriting addressed to Immortal Beloved. It is a romantic love letter, but the identity of the Immortal Beloved is never revealed.
This mystery has intrigued musical sleuths for two centuries. There are many contenders for the title, but no definitive answer. It is a mystery Beethoven took to his grave. Since the letter was never delivered, the object of his affection herself may have never known.