What happens when you tear a muscle and why does it hurt?

Tearing muscle: Tearing a muscle involves ripping apart muscle fibers in a muscle bundle, along with tearing associated vascular supply and nerve supply. It hurts due to the trauma of the injury causing irritation and inflammation in the muscle, nerve and vascular structure, along with the swelling and bleeding into the tissues.

Muscle fibers break: A muscle strain is when some muscle fibers get stretched too far and/or some fibers tear. People generally call it a tear if enough fibers break, leading to some bleeding and swelling inside the muscle, resolving later with some healed scar tissue and some tightness in the area (less elastic). Any torn muscle, skin, fat, etc... Releases chemicals signaling pain. Resultant inflammation also hurts.