Michael's never been one for nostalgia and celebrating. Really, who wants to commemorate the first time they were slapped so hard their head hit the wall? Or the first car stolen when it was to take their little brother to the emergency room? Maybe it would have been different if there'd ever been a time that Mom had stepped in, or stepped up and taken him and Nate away from that asshole. Then he might have an appreciation of events that should be celebrated.
Instead, there's very few lists of firsts that he proudly remembers. First kills. First payoff to world-known terrorist. First missile site destroyed. Even the first girl he ever really loved has a habit of leaving a wake of fire and rubble in her path.
Even the one day that he should celebrate rings bittersweet. Sure, the day after the last day of high school, he got to leave all the pain behind. But he also had to leave Mom and Nate.
Of all the pain his dad visited on him in childhood, that's the one transgression that Michael will never be able to forgive.