The reason Harry conjures a stag as his Patronus is because it's the animal that represents his father, James Potter.
Here's why this is significant:
* Family Connection: The Patronus is a powerful magical defense against Dementors, but it also reflects the wizard's deepest, happiest memories. For Harry, the memory of his father is one of great joy and strength, even though he never knew him well.
* Paternal Figure: Harry desperately seeks a connection to his father and the stag Patronus is a way for him to feel that connection even though James is gone.
* Symbolism: The stag is a majestic and noble creature, symbolizing the qualities Harry admires in his father: bravery, loyalty, and love.
This scene is pivotal in the story, showing Harry's growing connection to his heritage and his ability to confront his darkest fears with the power of love and happy memories.