Why couldn't Leon understand that Matt's passing hurt him, too? Sure, Marcus didn't know Matt nearly as long as Leon did, and they didn't have the same close relationship, but he was still a friend. And Marc's heart was shattered, too. However, what made it worse was Leon completely shutting off any semblance of humanity within him. He was hiding from his pain instead of sucking it up like a man. Marc didn't have that luxury. He was stuck with his emotions, and now, his loneliness. It was as if he didn't have Leon anymore. He didn't share the same intimate connection with the vampire. He lost both Matt and Leon that day. They were both dead. 

This was his moment. This was his time to shine. This was the day he was going to prove to everyone that he could do this, that he wasn't a failure. But, it was hard to do that when his copilot kept thinking about how much Marc was going to fail. 

They were inside the Iron Shock, being airlifted out to the ocean to fight the Kaiju that appeared nearby. He forgot what they called it. But that didn't matter. What was important was that he managed to take it down. But, being stuck in a drift with someone who was a complete downer didn't help. It was easier to internalize Leon's feelings when his thoughts kept taking over his mind. He tried to express this to the other man through positive thoughts. I can do it. You know I can. The second test run was successful. But those thoughts weren't getting through. If Leon didn't get over this soon, they'd both die, and millions of people on the Los Angeles coast.