The human life span is small, to say the least. They last only about eighty years. In comparison to the long time humans have walked the Earth, it is very small indeed. In life human’s really have no way to be permanent or immortal.
Humans have the capacity to realize their end. It is a unique quality that they have, to realize that they, as a race, barely brush the surface to Earth’s history. With this humans have created a sort of general acceptance of this fleeting existence.
In a smaller sense there is a much simpler thinking to it. People do not often go to lengths to accept the fact that what they do in this life does not matter. In fact, they often have a glorified ideal of their names remembered through the ages. Instead of understanding that sooner or later everything gets washed away, people want to believe that their memory or theiressence will live on.
So what they do instead is look beyond their lifespan and try to create a sort of legacy when they go. Something so they will be remembered. A book, a song, a note to your spouse, something so that they will not be forgotten. That is where human’s immortality lies. That is where their hope lies; in their legacy.
This gives people hope so they can live forever. In a way, Gilgamesh is immortal. He has died but he lives on in his tales. People still recognize him as a person. His quest for immortality did not fail, in fact it did quite the contrary, it let him be remembered forever.
That is why people become presidents, why they strive to be good at anything. They want to be good because they want to live on. They want to be remembered as the founder of the successful company, the builder of the tallest building. All these things, even if they now have social implications thrown in, are done for their legacy.
If this wasn't the case, what would drive humans to outdo each other? For the sake of competition? Then where did this need to be better come from? People would not want to better themselves if they knew it was going to disappear once their eighty years of life. People want to be recognized for their achievements. This makes people feel safe somehow, because humans are always worried about being forgotten.