Mortarion was even more thin-skinned than Perturabo, but what he had over Perturabo was that he could listen to reason—whether he changed afterward was another matter.
Though he was a Primarch, Mortarion was more like an ordinary man simply trying to live his life. Back on Barbarus, if Typhus had not constantly urged him to rebel, he might really have stayed in that little village, farming the land and protecting it from harm. This trait became especially clear after he fell to Nurgle: every day, he muddled along and grew rotten plants on Barbarus. Only Typhus never stayed idle, stirring up trouble throughout the Imperium in hopes of winning Nurgle's favor.
But a Primarch was still a Primarch. Limited editions were always more collectible than mass-produced goods. Besides, Mortarion's slacker nature suited Nurgle's tastes quite well, which was why he had always held a place in Nurgle's heart.
Mortarion was difficult. He had a bad temper, was easily provoked, then sat off to one side stewing once he had thought things through, hating others for being too crafty while hating himself for never learning his lesson. Usually, he seemed taciturn, but his mind was full of little thoughts. He responded to gentleness, not force, and even when he knew he was wrong, he would never admit it.
To put it more aptly, if the two of them played a game, The Emperor was top rank while Mortarion was bottom tier. The Emperor would stand beside him, offering commentary. Even if Mortarion knew he was right, all the enjoyment drained out of the game. At night, the moment he closed his eyes before bed, he would remember The Emperor's remarks, toss and turn until three in the morning, then wake up the next day finding everything about The Emperor irritating.
Perhaps