The reason why Zhuge Liang die from exhaustion is because Zhuge Liang had never took any breaks when there was war going around. In other words, he always up late planning on how to make the Shu Kingdom expand with more territories. He always focused on how to win over the land when he never thinks about anyone else until he finally realized what he had done before his death. He had realized that the people that had sacrificed to help expand the Shu Kingdom. Poor Zhuge Liang had to scarified his time just to plan things for Liu Shan. There is also another reason why he was exhausted is because of all these stress going into him from Liu Shan. I agree that Lui Shan isn't perfect like his father is because he was told by his father to listen to what ever Zhuge Liang say or what to do. That's why pressure had went into Zhuge Liang and caused him stress to help the Shu Kingdom win every expansions. But when he came to face the strategist general from the Wei Kingdom(Sima Yi) he was being pressured more since there was another strategist in the battlefield. Zhuge Liang was able to win during some fights with Sima Yi is because Sima Yi didn't know what Zhuge Liang was doing until they fight once more at the Wuzhang plains. That's when Sima Yi had pushed Zhuge Liang into more pressure. Sima Yi knows what Zhuge Liang was planning from the start since Sima Yi was award of Zhuge Liang's suspicious tactics on how Zhuge Liang had carefully plan on ambushing their enemies from the other times. That's why Sima Yi had planned a counter tactics for that situation.