I
"Hashem called to Moshe" (Vayikra 1:1). The aleph of "Vayikra" is small, representing Moshe's unsurpassed humility (Bamidbar 12:3). Vayikra is a term of love, a term the angels use "one will call another and say" (Yeshaya 6:3). By contrast, when Hashem spoke to Bilam, the term "vayikar" is used (Bamidbar 23:4), implying chance and impurity (Rashi).
Hashem loved Moshe precisely because of his humility. Bilam was arrogant (Avos 5:22), and, therefore, is abominated by Hashem (Mishlei 16:5). The missing aleph indicates his lack of humility.
Moshe was chosen because of his good heart. He saw the burdens of his brothers, and focused his eyes and heart to be distressed over them (Shemos 2:11, Rashi). He grazed Yisro's sheep (ibid 3:1) with extraordinary care and concern, thus deserving to be the trustworthy shepherd of Am Yisrael (Shemos Rabbah 2:2). He was reluctant to accept his mission, because he did not want to take a position greater than his brother Aharon (Rashi 4:10).
Parshas Hachodesh, typically read before or on Parshas Vayikra, begins, "Hashem said to Moshe and Aharon" (12:1). Aharon, too, had a good heart, and was happy about Moshe's appointment (4:14, Rashi). R' Elazar ben Arach said a good heart is the most important trait, and his rebbe, R' Yochanan ben Zakai, concurred (Avos 2:13).
R' Elazar ben Arach was the greatest of the five students of R' Yochanan ben Zakai (Avos 2:12). He came to an area of Northern Israel, known for its wine and springs, natural pleasures which had deprived Am Yisrael of the ten tribes (Shabbos 147b). They were preoccupied with physical pleasure, didn't learn Torah, and abandoned mitzvos (Rashi), leading to their exile. Incredibly, R' Elazar ben Arach was pulled by these pleasures, and he forgot his learning. When called to read "Hachodesh hazeh lachem" (12:2), he read instead "Hacheireish haya libam - their heart was deaf." How did it happen and what does this mean?
Yalkut Shimoni (Koheles 973) provides the background. R' Yochanan ben Zakai had five students. As long as he was alive, they sat before him. When he died, they went to Yavne, but R' Elazar ben Arach went to his wife in a place where the water and the view were pleasant. He waited for his friends, thinking they would come to him. When they didn't come, he wanted to go to them, but his wife wouldn't let him. She felt that since he was the greatest student, the others should come to him (See Koheles Raba 7:7). Without the interaction with his friends, he forgot his Torah.
This underlies the Gemara's conclusion. R' Nehorai (possibly R' Elazar ben Arach himself) said, "Exile yourself to a place of Torah, and don't say it will come after you. It is your friends who will cause it to remain with you. Do not rely on your own understanding."
II
This explains how it happened, but what does the change from "Hachodesh" to "Hacheireish" mean? My son R' Moshe suggested as follows. Yisrael count months by the moon. The moon receives the light of the sun and reflects it. So too, Am Yisrael received the Torah from Hashem and Moshe, and not must continue to receive it from one another.
The letters gimel and dalet stand for g'mol dalim, help the poor. The leg of the gimel extends towards the dalet, as the giver runs after the poor. The leg of the dalet extends towards the gimel, as the poor person presents himself to his benefactor to spare him from the bother of pursuing him (Shabbos 104a). Thus, the dalet of "Hachodesh" represents the moon-like quality of receiving from others.
Even the greatest Torah scholars need to receive knowledge and stimulation from their peers and students (Ta'anis 7a). R' Elazar ben Arach lost these crucial interactions and forgot his Torah. His heart became deaf, unable to hear from others. The dalet rounded into a raish, as he did not extend himself to receive from others.
The small difference between dalet and raish can lead to polar opposites. The large dalet of "echad" in Shema represents our recognition that we receive everything from Hashem Echad. The large raish of "Do not bow down to el acher" (23:14) signifies a denial of this recognition (Vayikra Raba 19:2).
R' Meir was warned not to miss or add a letter. He responded that he was careful even about a dalet becoming a raish, which would destroy the world (Eruvin 13a), presumably based on the Midrash of Echad and Acher (See Rashi Sota 20a).
The Gemara (13b) associates R' Meir with R' Nehorai and R' Elazar ben Arach. Rebbe added: I am sharper than my peers (me'chavra'i) because I saw R' Meir from behind. Seeing from behind can mean the knot of the tefilin (Rashi 33:23), shaped like a dalet. This represents learning from others, which Rebbi did when he gathered all of his colleagues to edit the Mishna (Rambam, Introduction to Mishne Torah). This word me'chavarai can mean from my friends as well as than my friends.
R' Meir, like R' Elazar ben Arach, was the greatest of his generation. Nevertheless, the halacha was not like R' Meir, because his friends could not understand him. R' Meir, who learned from Elisha ben Avuya, who known as Acher (Chagiga 15b), was referred to as Acheirim (Tosafos, Sotah 12a). He didn't interact well enough with others. When he distinguished sufficiently between dalet and raish, he became R' Nehorai, taught and learned from others, and halacha followed him.
The lesson of humility, derived from Moshe in Vayikra, is reinforced by the inclusion of Aharon in Parshas Hachodesh. The incredible story of R' Elazar ben Arach, and his associations with R' Meir and R' Nehorai, can be understood in a similar vein. We must receive Torah, moon-like, or risk losing our Torah with a deaf heart. We should all strive to be good-hearted, the most critical character trait of all.