Again, from Tsongkhapa:
Although we all have the thought that at the end of our life will come our death, each day we think, “I will not die today” and “Today too I will not die.” In this way, right up to when we are about to die, our mind holds on to the idea that we are not going to die.
If you do not take to heart an antidote to this, if your mind is obscured by such an idea and you think that you will remain in this life, then you will keep thinking about ways of achieving happiness and eliminating suffering in this life only, thinking, “I need such and such…”
Devoid of the perception of death, addicted to thinking that he will never die, the human being deprives himself of his essence, prevents the notion of the most important from blossoming within him. He is distracted by perishable futilities, wasting his time deluding his spirit. If for a moment he understands the true nature of death, he will no longer be able to live as before, no longer accept to get lost in worldly banalities, and will demand, even at the cost of his life, a reason that justifies his reality. Since there is death, since death annihilates the body, forces a final separation of possessions and relations, what is left? Is there anything left? Searching for answers, the being transforms his behavior and cancels the dangerous notion of “I will not die today”, moving on to the obsession: “If I die today… what then?”.