Why doesn't the proton decay? The Standard Model predicts proton decay via grand unified theories, yet the proton has never been observed to decay. This paper derives the proton's stability from the structure of Pentagon Physics. Eight stability layers protect the proton's eigenmode configuration from unwinding: Galois boundary confinement, phase balance, spectral gap, topological winding number, symmetry protection, Lagrangian minimum, thermal ratchet, and the mass gap. The tunnelling suppression across all eight layers produces a stability lifetime vastly exceeding the age of the universe.