Neal A. Maxwell
"Denying the doctrine of the premortal existence of man shrinks man's perspective. He begins to think, mistakenly, that this life is all there is; that the insignificant 'me' of a tiny 'now' is not only all there is, but all there ever was. The adversary is quick to use the 'what if' there is no purpose to life in order to induce some to act 'as if' such were the case. The resultant misbehavior only deepens the despair (see Moroni 10:22).
"Naturally, such a view tends to be accompanied by a diminished belief or a pronounced unbelief in the resurrection and a perpetuation of personality, which pushes a person's hope for the future down to nil. This 'no-answer' attitude equates with a 'no-answerability' concept that too often leads to the 'eat, drink, and be merry' outlook. Thus one-dimensional mortality relentlessly promotes a one-dimensional morality!" (But For a Small Moment, p. 89)
Joseph F. Smith
"Some people cannot think of anything else but annihilation. What a glorious prospect for the sinner! Then he could say, 'Let us eat, drink, and be merry, for to-morrow we die, and next day we will be annihilated, and that will be the end of our sorrow and of God's judgment upon us.' Do not flatter yourselves that you are going to get out of it so easy. This Book of Mormon is replete all the way through with the testimonies of the servants of God that men are born to be immortal; that after the resurrection their bodies are to live as long as their spirits, and their spirits cannot die. They are immortal beings, and they are destined, if they commit the unpardonable sin, to be banished from the presence of God and endure the punishment of the devil and his angels throughout all eternity. I think the wicked would prefer annihilation to the suffering of such punishment. That would be an end to punishment--an end to being. This view cannot be reconciled with the word of God." (Collected Discourses 1886-1898, ed. by Brian Stuy, vol. 4, Joseph F. Smith, Jan. 20, 1995)