Voldemort burns when Harry touches him because Harry possesses a piece of Voldemort's soul, which was unknowingly transferred to Harry when Voldemort tried to kill him as a baby. This connection makes Harry a Horcrux, a magical object containing a part of Voldemort's soul.
Here's why this connection causes Voldemort to burn:
- Horcruxes are inherently unstable: Creating a Horcrux is a dark and dangerous act, and the act of splitting one's soul leaves it fragmented and unstable.
- The connection between Harry and Voldemort: The piece of Voldemort's soul in Harry is linked to the rest of his soul, creating a powerful connection.
- Harry's love for life: Harry's love for life and his desire to protect others is the opposite of Voldemort's hatred and desire for immortality. This fundamental difference creates a conflict when their souls come into contact.
When Harry touches Voldemort, the instability of the Horcrux, combined with the powerful connection between their souls, causes a magical reaction. This reaction is manifested as burning, a visual representation of the conflict and destruction happening at a soul level.
Ultimately, the burning symbolizes the vulnerability of Voldemort's soul and the power of love and life over darkness and hatred.