Of course, every relationship is defined by the last interaction of the parties.The last time they spoke alone Cooper told him to GTFO. I think Roger is the only one he meant something to. That can't be it.What dream?Don greets Roger with 'Cooper still dead?' in his hallway, and after the dream he gets sentimental for a guy who had nothing to do with his career?I'm going to out on a limb and say Don looked "down" because Bert died. I know it is complex but that's my theory.Don looking down at the end? Was Bert calling him a sell out? He realized at that moment he's divorced again and alone?
I think it just finally hit him that Cooper really was gone. The news of Cooper's death came at a time when Don was being bombarded by a lot of other things...his marriage falling apart, being forced out, the moon landing.
And Copper had a lot to do with his career. Hell Don wouldn't have a career if it wasn't for Cooper. Remember when Campbell ratted out Don/**** to Cooper and Cooper basically just shrugged?
Roger had to talk him into it because Don doesn't want to work for a place like McCann. I think he even referred to McCann as a "sausage factory" once. Early in the series Hobart did everything (including offering him a crapton of money) but blow Don in order to steal him away from SC. And at the end of Season 3 Don let the plot to get away from McCann which lead to SCDP. You really think this was all about money to Don?