"For he who is least among you all--he is the greatest." (Luke 9:48)
"Man is destined to die once, and after that to face judgment." (Heb. 9:27) I will die someday, so will you. Being relatively young, it is hard for me to actually realize this. When you can grasp that you are mortal and eventually you will die, you can realize the importance of focusing your life on serving God with everything while you have the chance. You can begin to see the importance of living holy lives in light of eternity.
James says that our lives "are a mist that appears for a little while and then vanishes." (Ja. 4:14) Compare the long life of a 100 years to 100 billion years. This is nothing compared to eternity in heaven or hell, which is beyond what our mind can comprehend.
When you compare our life to this amount of time spent after our physical bodies die, you realize that our life is indeed a mist. If not less than that. It is so short, yet our life decides where we will be for a incomprehensible amount of time. For infinity to be exact. Because of this, we should "work out our salvation with fear and trembling" (Philippians 2:12), knowing that this is the only life we get and it is the life that determines our eternal destination.