Why it happens
As meat cooks, its muscle proteins tighten and squeeze out moisture. Cooked gently and not too far, enough juice stays inside to keep it tender. Overcooking tightens the proteins so much that most of the moisture is lost and can't be put back. Lean cuts such as chicken breast, pork loin and lean steak have little fat to help them seem juicy, so they dry out fastest. Cutting meat straight after cooking, without resting it, also lets more juice run out.