Valentine is kind and beautiful and devoted. Her father loves her, her grandfather loves her, and so does pretty much every other man around. Here's the Dumas's description of the crowd at her (fake) funeral:
Almost all were young men who had been forcibly struck by Valentine's death and who, despite the cold mists of the century and the prosaic spirit of the age, felt the elegiac poetry of this beautiful, chaste, adorable young woman, struck down in her prime. (105.6)
To top it all off, she appreciates their love. She's a perfect little girl, and, frankly, perfection is boring. At the end of the day, she is kind like Juliet of Shakespeare's Romeo and Juliet. She's young, innocent, and deeply in love with Maximillian. In a world full of revenge and bitterness, she is a breath of fresh air. Plus, her name makes us think of little candy hearts.