According to an article by Donald Parry, the writers of ancient scripture often contrasted one idea in one line or stanza with an opposite or anti-thetical idea in a parallel line or stanza. Alma 36:21 illustrates this principle, where Alma speaks to his son Helaman:
Yea, I say unto you, my son, that
there could be nothing so exquisite and so bitter as were my pains.
Yea, and again I say unto you, my son, that on the other hand,
there can be nothing so exquisite and sweet, as was my joy.
[Donald W. Parry, "Antithetical Parallelism in the Book of Mormon," in Reexploring the Book of Mormon, F.A.R.M.S., p. 168]