I was recently asked this good question: "Why must we strive for eternal life? Is one lifetime not enough?" I'm glad to see someone thinking rationally about our mortality, because too many people choose to ignore it, thinking and living like they will never die. As for answering the question on why we should strive to live forever after having lived once, I pointed this inquirer to a few things the Lord Jesus has to say about this, and I want to share them here.
First, the choice of whether to live forever or not is not a choice of immortality versus annihilation (not existing anymore). It is a choice between the eternal life of salvation and the eternal death of judgment. Everyone is actually going to live forever whether they want to or not. The question is what state - what destiny - we are going to live in:
24 Truly, truly, I say to you, whoever hears my word and believes him who sent me has eternal life. He does not come into judgment, but has passed from death to life. 25 “Truly, truly, I say to you, an hour is coming, and is now here, when the dead will hear the voice of the Son of God, and those who hear will live. 26 For as the Father has life in himself, so he has granted the Son also to have life in himself. 27 And he has given him authority to execute judgment, because he is the Son of Man. 28 Do not marvel at this, for an hour is coming when all who are in the tombs will hear his voice 29 and come out, those who have done good to the resurrection of life, and those who have done evil to the resurrection of judgment. (John 5:24–29)
Second, the choice to strive for eternal life is the choice to strive for what will last forever in the abiding blessings of God versus what will perish in the passing pleasures and possessions of this world:
27 Do not work for the food that perishes, but for the food that endures to eternal life, which the Son of Man will give to you. For on him God the Father has set his seal.” 28 Then they said to him, “What must we do, to be doing the works of God?” 29 Jesus answered them, “This is the work of God, that you believe in him whom he has sent.” (John 6:27–29)
Third, eternal life is the purposeful and providential will of the Heavenly Father for all that will come to His Son. He does not want them to only live one passing, shriveling life, but an ongoing, persistent, thriving life. This is what He sent His Son, Jesus, to accomplish:
37 All that the Father gives me will come to me, and whoever comes to me I will never cast out. 38 For I have come down from heaven, not to do my own will but the will of him who sent me. 39 And this is the will of him who sent me, that I should lose nothing of all that he has given me, but raise it up on the last day. 40 For this is the will of my Father, that everyone who looks on the Son and believes in him should have eternal life, and I will raise him up on the last day.” (John 6:37–40)
The thief comes only to steal and kill and destroy. I came that they may have life and have it abundantly. (John 10:10)
Fourth, eternal life is not merely about living forever, but about knowing and loving God forever as we live with Him:
1 When Jesus had spoken these words, he lifted up his eyes to heaven, and said, “Father, the hour has come; glorify your Son that the Son may glorify you, 2 since you have given him authority over all flesh, to give eternal life to all whom you have given him. 3 And this is eternal life, that they know you the only true God, and Jesus Christ whom you have sent. (John 17:1–3)
24 Father, I desire that they also, whom you have given me, may be with me where I am, to see my glory that you have given me because you loved me before the foundation of the world. 25 O righteous Father, even though the world does not know you, I know you, and these know that you have sent me. 26 I made known to them your name, and I will continue to make it known, that the love with which you have loved me may be in them, and I in them.” (John 17:24–26)
No, one lifetime is not enough for me - not one only lived in this world - not when I can live with God in His new heavens and new earth, forever seeing His face, never again shedding a tear because there will be no more death, mourning, crying or pain, since He will have made all things eternally new, forever refreshed by His river of life (Revelation 21-22). At the risk of sounding trite, an eternal lifetime with Him is all I'm living for - you and I can live for nothing more.