Why did boaz have to marry Ruth?

Boaz had to marry Ruth primarily due to the Levirate marriage custom in ancient Israel, which required a close relative to marry the widow of a deceased family member to preserve the family line. Ruth was the widow of Boaz's relative, Mahlon, and by marrying her, Boaz ensured that Mahlon's lineage would continue. Additionally, Boaz was motivated by his admiration for Ruth's loyalty and character, as she had committed herself to care for her mother-in-law, Naomi. This union ultimately led to the birth of Obed, the grandfather of King David, linking Ruth to the lineage of Jesus.