"You want me to have a secret meeting with the Son of the Sword?" "I must be high from the morphine." "You know, stealth isn't exactly my strong suit." "He's very clumsy."
JOSH LYMAN AND DONNA MOSS
Memorial Day (5x22) | The West Wing
It must have enraged Lucifer that Sam has a brother who shows up for him. Just a few moments before Dean arrived, Lucifer tried to talk Michael down and say they didn’t have to fight. They could just be brothers. Michael said no. Lucifer’s betrayals were too great.
Like we focus on Dean and the impala catching Sam’s attention inside, but I do have to wonder if Lucifer also lost focus and accidentally gave Sam an opportunity to come to the surface, because it filled him with jealous rage that Dean showed up for his brother in spite of everything.