What I love about this scene is how it shows how much Chief cares for Cortana. He’s literally spent the most part of his life following orders from his superiors - doing everything they say, making sure they were content with his work and his outcome. He’s always done his best - not necessarily wanting to please but wanting to get the job done. He was built to follow orders and do his job - taught from a young age that that was the way of life and the only way. But when Cortana’s life comes into the question, those years of sticking to the regulations and saying, “Yes, ma’am/sir” every single time he’s given an order goes out the window. Cortana is more important to him than his whole purpose in life.
"I was Squad Leader. The last mission was therefore my responsibility… and members of my squad died. What did I do wrong?” "It is acceptable," Mendez said, "to spend lives if necessary." He finally turned and met John’s gaze. "It is not acceptable, however, to waste those lives. Do you understand the difference?" "I… believe I understand, sir," John said. "But which was it on this last mission? Lives spent? Or lives wasted?" Mendez turned towards the blackness of space and didn’t answer.