If a person is of excellent mind and character, separates himself from the behavior of the masses, and attaches his mind to the Holy One, blessed be He, a spirit of holiness rests upon him. Some character traits, like anger and sadness, prevent prophecy; and sometimes God decides to withhold prophecy from a person even though he would have been worthy of it.

Prophecy: A fundamental principle of faith:

Knowing that God grants prophecy to human beings is a principle of faith.

Characteristics of prophets and preparations for prophecy:

Prophecy only rests upon a person who is extremely wise, has mastery over his inclinations so that his drives do not overcome him in any way, rather, he always overcomes his natural drives, and upon one who possesses an extremely broad and sound mind.

A person who has all of these characteristics and is sound in body, when he enters the orchard [pardes] and is drawn after those great and obscure matters and has a mind that can understand and grasp them, he becomes continuously more holy and separated from the ways of the masses who walk in the darkness of temporal life. He constantly energizes himself and trains himself not to think about any useless matters, frivolities, or schemes. Instead, his mind is always free to be connected above, bound up with the heavenly throne, striving to understand pure and holy forms. He views the world through all of the Holy One, blessed be He’s wisdom, from the primordial form until the core of the earth, and from there recognizes His greatness. Immediately, the spirit of holiness rests upon him.

The prophet is elevated and transformed into a different person:

When that divine spirit of holiness rests upon him, his soul merges with the level of the angels who are referred to as ishim and he transforms into a different type of person. He understands that he is no longer as he was, but has risen above the level of other wise men.

Things that prevent prophecy:

Some character flaws, like anger, totally prevent prophecy. The Sages said: If a prophet gets angry his prophecy abandons him (Pesaĥim 66b)…. Anxiety and anguish [likewise prevent prophecy]…. Our Sages expressed it this way: “Prophecy does not rest upon one who is in a state of laziness or sadness. Instead, it rests upon one who is in a state of joy” (Shabbat 30b).

Those who are precluded from prophecy:

We believe that there are those fit for prophecy, but when they prepare themselves for it do not prophesy, and this is in accordance with the divine will. In my view, prophecy is similar to miracles and functions like them. For if it was a natural phenomenon, then anyone who was fit for prophecy by nature, and trained himself based on how he was educated and instructed, would necessarily experience prophecy…. However, it is impossible, in our opinion, that one of the ignoramuses among the masses could experience prophecy any more than a donkey or frog could. Our principle is: Training and personal perfection are necessary, and then it is possible that the divine ability will be manifest in him. (Rambam, Mishneh Torah, Hilkhot Yesodei HaTorah 7:1; Shemoneh Perakim, chap. 7; Guide of the Perplexed II:32)

Further reading: For more on anger and how problematic it is, see p. 194; A Concise Guide to the Sages, p. 421.