First, let me establish a fact about human existence: we long for the satisfaction that comes from extremes. Knowing that by "extreme" I mean the end of any given spectrum, examples of this satisfaction include the "fresh start" of a beginning, the finality of an end, the assurance of holding an idealistic political view, or winning an award. As a part of this, we long to resolve opposing extremes, as happens when two people become friends, when an argument is resolved, or when you watch a crossover (like the Avengers).
However, this next quotation (from the concerned message) suggests this quest may be problematic:
"We may feel we are at the beginning or end of our lives, but when we look at where we are against the backdrop of eternity—when we realize that our spirit has existed for time beyond our capacity to measure and, because of the perfect sacrifice and Atonement of Jesus Christ, that our soul will exist for an eternity to come—we can recognize that we are truly in the middle."
There is no such thing as a temporal extreme. We may feel that a graduation is "the end" or that a marriage is "the beginning", but ultimately they are transparent phantoms through which you can see infinitely into the future or the past. But there's more:
"For it must needs be, that there is an opposition in all things. If not so, my first-born in the wilderness, righteousness could not be brought to pass, neither wickedness, neither holiness nor misery, neither good nor bad. [...]" (2 Nephi 2:27)
The desire for extremes is also a longing to get rid of conflict. Whether (as in politics) we want to utterly destroy the other side, or (as in friendship) to completely merge with it, our lust for extremity manifests itself as a need to eliminate the metaphorical "no-man's-land" between the conceptual foes. Knowing this, the above quotation makes us even more uneasy. "Opposition in all things" necessarily means that there is no such thing as an "unchallenged" extreme, that "resolution" is a fantasy. Since everything has an opposite, there will be no end of conflict between the extremities of any given spectrum, and no one will ever completely belong to either end.
Essentially, humanity is stuck between a rock and a hard place. Whether it be past and future, beauty and ugliness, or happiness and misery, humanity is wedged in the middle of two unmovable opposites. He will never completely partake of either side, and he will never reconcile anything. Grim, eh? So, what are we to do? The answer is simple: we must accept where we are, in the middle.
The doctrines I have elucidated are really very clever, as they force those who really believe in them to come to terms with the here and now. You see, thinking about our eternity makes all measurements of time insignificant. Because the future will never come, and because you will never "arrive" anywhere, you have no choice but to be content with what you have right now. Similarly, since you will always be between extremes, the only rational choice is to be content with being somewhere in the middle.
Living in the middle leads a person to be infinitely happier than trying to live on either side. Rather than expending your energy in the impossible quest after extremes, it is much better to be content where you are, as you will never be anywhere else.