אין בטובה למעלה מענג
ואין ברעה למטה מנגע
There’s no good higher than delight (oneg/ענג),
There’s no evil lower than a plague (nega/נגע). [*]
This teaching from the Sefer Yetzirah is based on the similar Hebrew roots of “delight” and “plague.” Both triliteral (3-letter) roots contain the same 3 Hebrew letters. The word “nega” appears in Va’yikra/Lev. 46 times — far more than in any other biblical book. Leviticus has much to do with “ritual impurity.” A “plague” like leprosy is a source of “ritual impurity.” “Ritual Impurity” disallows us access to worship in the Temple — i.e. to G-d’s Presence. The verse could then imply that “oneg” is the delight (i.e. delighting in what G-d does for us) that allows us closeness to G-d ; “nega” is what keeps us from G-d. They are opposite ends of what life offers, yet contain the same letters!
On the surface, it might be taken for the kind of word-play that often appears in the Talmud: A pun is used to teach a homiletic lesson with a degree of ironic humor.
But this teaching appears in the Sefer Yetzirah in the broader theme of G-d using the Hebrew letters as “agents of Creation.”  G-d creates the letters, which can then appear in the order N-G-O¹ (O¹ indicating ayin/ע) in “nega/plague” or as O¹-N-G in “oneg/delight.” The Hebrew letters themselves are the means by which G-d appears as either “oneg” or “nega.” The “name” becomes the “form.” A sameness of letters implies a sameness of essence.
The text says that G-d puts the letters in a wheel and rotates it as appropriate. In his version of the Sefer Yetzirah, Rabbi Aryeh Kaplan, z”l, has an illustration that depicts this wheel/circle and the Hebrew letters. 
But Rabbi Kaplan elsewhere makes another, perhaps crucial, point. In Hebrew, the 3rd person-singular of a verb can also be the imperative form.  Thus, the text “He [G-d] engraved [the letters]…”  can also be read as a command: “Engrave…,” which Rabbi Kaplan interprets as a direction for one step in the process of contemplating the letters. In the present verse (2:4) then, the text is telling us that we should “rotate” the letters.
From this, we might derive a deep cognitive lesson: The Source of all our experiences is ultimately the One Who is eternal and unchanging, but Who is expressed to us in varying forms through the Hebrew letters and their combinations. This being so, we ourselves can “rotate the wheel” of our perception and interpret events as “oneg,” even when seemingly confronted by “nega.” Their Source and essence are the same.
A similar idea is taught elsewhere when discussing “bitachon” — “trust (in G-d).” 
“When you are going through a difficult period in your life, remember that your challenge is coming from the One who loves you so dearly; and, therefore, it is ultimately for your good.” 
We might further say, after the Sefer Yetzirah, that the challenge is not only “from” the One, but is the One, expressing Itself to us as the challenge!
In this case, we “rotate the wheel” of our perception by “remembering” (deeply) the Divine Source of the challenge and Its essential Goodness.
Rabbi Morris Lichtenstein’s definition of “faith” is perfectly in line with this:
“Faith is the whole-hearted realization of the Divine Presence accompanied by the conviction of His profound goodness.” 
“Whole-hearted” implies the high degree of our sincerity, which we attain by affirming that the Source of all existence is nothing other than G-d’s own existence:
“All existence consists of the same substance…The whole universe is but a multiplicity of combinations of the same essence…The great varieties that we observe in nature mark only differences in form, not in essence. Primarily, One G-d [is perpetually bringing] them all into existence, and He resides in and expresses Himself through them.” 
Taken together, we learn that developing “bitachon” requires that we come to see all that exists as emanating (but never separate) from a single, unchanging Source, moderated in presentation through the Hebrew alphabet but without any change in Its essence, always present and always with inseparable goodness.
[*] more strictly, leprosy or some other loathsome affliction.
 Sefer Yetzirah 2:4
 Friedman, Irving, trans.; The Book of Creation; Samuel Weiser, Inc., © 1977; p. 6
 Kaplan, Rabbi Aryeh; Sefer Yetzirah – The Book of Creation; Samuel Weiser, Inc, © 1990; p.123
The note under the illustration incorrectly says that “nega” is obtained from “oneg” by “notation.” As is clear from the text and from R. Kaplan’s comments, it should read “rotation” instead.
 ibid., p. 107
 Sefer Yetzirah 2:3
 Friedman, Ms. Chana Toby; Mastering Patience (based on Sefer Erech Apayim); © November, 2010 by the author
 ibid., p. 35
 Lichtenstein, Rabbi Morris; Jewish Science and Health; Society of Jewish Science, © 1925; p. 135
 ibid., p. 11 “Multiplicity of combinations of the same essence” is precisely what the Sefer Yetzirah means by its discussion of the Hebrew letters.