Parshat Ki Tisa
The question was posed: Why did Moshe need to go up to the top of Har Sinai the second time for another full 40 days? He had just been there the first time when he learned the whole Torah, reached the spiritual highs and whatever else he accomplished up there. So when he had to go the second time why did it need to be for 40 days? It could have just been a day or two to get the new set of tablets. What was he doing there for 40 days the second time?
Neil from Modern Uberdox quotes the Sfas Emes to answer this question. The Sfas Emes explains a phenomenal answer. Briefly, he says that after the first time, Israel did a horrible sin and had gone through stages of repentance. The Torah of a baal teshuva is no tthe same as that of even a tzaddik. It is much greater. After having experienced all that happened, the first set he had already studied was no longer relevant to Bnei Yisrael. Ergo, Moshe had to go up a second time to study it all over again with a fresh approach.
I would like to suggest another answer. After Moshe came down the first time, he saw what the people had done. The passuk says he got angry and threw down and broke the tablets. We know and the mussar seforim tell us that anger is really the worst of all the bad charachter traits. Often it is based on haughtiness, though I would be hard pressed to say that about Moshe who the Torah testifies about as having been the most humble person ever.
Even if anger is not based on haughtiness, even if anger is justified, as it may have been in this case, anger is still destructive. I would like to suggest that because Moshe got angry, he "ruined" the effects of his having spent 40 days on Har Sinai learning the Torah. because of his anger, even though it was justified, he had to spend more time re-learning the Torah and refining his charachter.