Why does Don soften as the series progresses? (Spoilers)
Don started off as this big bad ass stoic baller type that douchey, libertarian, Fight Club fan boys seem to worship like a god. Don is openly antagonistic, outright insulting his fellow employees and acquaintances, his philandering and unscrupulous business sense in the first two seasons (especially start of season 1) but by he seems to become so mopey, sulky and actually somewhat more considerate as he advances at the ever-shifting chimera that is Sterling-Cooper-Whatever-Whatever.
I feel like maybe this change in personality kicks in when his brother commits suicide, resulting in his marriage to Betty collapsing as his real identity is revealed.
For whatever reasons it happens though it just seems so unrealistic and also like lazy characterization on the part of the writers.
Don is a completely different character from the pilot to even how he is in season 4, it almost seems like he makes a return once, when he leave's Joey's poster in the taxi cab (can't remember what season). I get that this is television and it's an elongated, serialized formatted which would mean subtly changing character arc, but It just seems hard to buy.