Why does cooking meat at low heat keep it juicier?

When meat cooks at high heat, the muscle fibers contract violently and squeeze out moisture rapidly, while proteins on the surface seize up quickly and can trap steam that forces juice out. Low and slow cooking allows muscle fibers to contract more gently and gradually, releasing less moisture overall. More importantly, low heat gives collagen time to slowly convert into gelatin, which coats the meat fibers and contributes to a perception of juiciness. The internal temperature also rises more evenly, so you avoid overcooking the outer layers while waiting for the center to finish. Staying closer to the target temperature means less total moisture loss, since every degree above roughly 160°F causes progressively more liquid to be expelled from the protein fibers.