Why did Selene ask Zeus to grant endymion eternal sleep?

Selene asked Zeus to grant Endymion eternal sleep because she wished for him to remain eternally youthful and beautiful, preserving their love without the ravages of time. By putting him into an everlasting slumber, she could visit him each night, ensuring they remained together, even if only in her dreams. This act reflects Selene's deep affection and desire to protect Endymion from mortality.