We don't perceive our lives as short-lived internally (assuming we age over 70) despite fear of death, a fear which has evolutionary founding.
We can however contemplate, given externally-acquired knowledge of our existence in relation to human existence, living existence (on earth), and the existence of the universe and come to the conclusion that life is short.
Further, with greater living standards and satisfaction of life, comparative to past human civilizations in a state of hunger, lacking any justice in their legal systems, few rights, little to no health care options, constant war/skirmishes, governmental stability, non-sophisticated economic system, no leisure time, very few individual rights, ect. ect. ect.
With that said, there are still conflicts, but there was essentially no potentiality for anyone aside from aristocrats or monarchs to achieve a degree of satisfaction. Well, Machiavelli would argue as well that these leaders were often foolish in this respect, usurping all pleasure from the populous which will inevitably lead to a revolution and chaos.
With that greater living standard, there are higher exit costs in death. Thus, more value in life. Once a resource is deemed more valuable, we have greater demand for it, and perceive the time we do have as insufficient to our desires.
Not to say those in the past would pass up immortality. There were just lower exit costs, especially if an individual had religious belief in an afterlife.
Prior to advanced human dialect, it's fair to say exit cost were even lower. As a person aged in a hunting and gathering society, the band would be weaker, more vulnerable to other bands as well as predatory animals. Not to mention, resources for the strong and able bodied would be "wasted" on a person that wouldn't be able to contribute. For those reason, we developed a capacity to be fine with death innately, since it's against our nature to desire to act in a way that would jeopardize our own progeny. Actually, at a certain point, there is actually an exit profit to death!
I'm not sure about my argument though...I have never really though about the perception of longevity of life over time.
EDIT:
My bad. I thought this was in D&D.