Here's why:
* The Mirror shows your deepest desire: The Mirror of Erised is a magical artifact that shows a person their deepest desire.
* Harry's deepest desire: Harry's parents were killed when he was a baby, and he longed to see them again. This longing was his deepest desire.
* The reflection: Harry saw his parents in the mirror, alive and happy, along with himself.
It's important to note that the mirror doesn't show the truth, but rather a reflection of the person's deepest desires, which can be misleading and even dangerous.