Optimus Prime sometimes gives the impression of grimness. Command Weighs heavily on him; though he has never hesitted in the fight to recover the AllSpark and restore Cybertron, he feels every soldier lost like a blow.  Eons of war have left him stepped in memories of lost friends and comrades long gone. It is their memory tht drives him on, and keeps him always dedicated to his principle of preserving freedom not just for the inhabitants of cybertron, but for all creatures across the cosmos.