The longer answer
A mount is a sequence, not a single motion. The gun starts at a low ready — muzzle up, butt below the armpit. The lift carries the butt up the ribcage toward the shoulder pocket. The roll seats the butt into the pocket as the cheek comes down onto the comb. The final position has the cheek welded to the comb, the dominant eye centred over the rib, and the muzzle already pointing where you want to shoot.
The same mount has to repeat round after round, bird after bird, with no thought required. Every other skill in the sport — lead, hold point, follow-through — assumes the gun arrives at your face in the same place every time. Mount is the foundation; everything else is built on it.