The general theme in the Fate Universe has always been that the antagonist or the "perceived villain" are the ones who seek immortality or believes immortality to be a benefit to humanity. While the protagonists have always been those who oppose it or believe it to be a detriment to humanity.
Fate generally argues that using Magic to grant humans immortality would be akin to cheating, or halting their growth, or is just bad. For reasons as trite as because it is not natural.I completely disagree with Fate's view on immortality.
In The Epic of Gilgamesh, the only reason that Gilgamesh (and mankind) didn't become immortal was that the Herb of Immortality that he had obtained was eaten by a snake. But in Fate, it was said that Gilgamesh "went back and obtained the Herb of Immortality again just for the sake of collection" but didn't use it because blah blah. Total BS and out of character for Gilgamesh! To Gilgamesh, he did not only fear death because he loved living, it was also because he wanted to observe humanity's path until its very end, so he saw death as an end to his duty as humanity's guardian. In Fate/Zero, he agreed to participate in the HGW because, in his words, Tokiomi had summoned and given him a body/a second life ("I treat him as my vassal and he supplies me with mana"), and that he later let Tokiomi be killed when Tokiomi planned to kill him. In UBW ending, we see that Gilgamesh stubbornly clings to life. It's because he hasn't fulfilled his goal or it's because he wants to see Kirei's path, etc. For whatever reason, for whatever purposes you see him cling to life, it shows that he still sees purposes in life and never wants to die. So, if Gilgamesh successfully obtained a method for immortality, he would no doubt use it! "Oh I'm over death" is an excuse to insert the "immortality isn't good" notion into the work - this is the authors' view - and not a representation of how the character should be. Gilgamesh in The Epic became more human because he had failed to achieve what he wanted, failed to overcome his human limitations. His brief sense of accomplishment and joy vanished just as quickly as he obtained it - that made him realize the full extent of his humanity, which was the final crucial step that completed him as a man and connected him with his people. The grief of Gilgamesh, the questions that death evoked and his quest for immortality resonate with every human being who has wrestled with the meaning of life in the face of death. Also in The Epic, Gilgamesh could not achieve the kind of immortality that was available to gods. Fate - by having him able to obtain immortality but turn it down by his own will - negates the whole meaning of his journey. Fictional authors have been using characters to speak their own minds, and anti-immortality is the message that Fate wants the audience to believe.
Gilgamesh, Alucard, Sun Wukong, etc. - all these characters who strive for immortality have an interesting thing in common: They all share a great fear of death. That fear alone motivated them to master great deeds. Their desire for immortality and power was also motivated by the fear of losing what they had. This is a very HUMAN desire, considering that immortality has always been humanity's greatest dream!
There's nothing "unnatural" about immortality - just as there's nothing unnatural about the advancement of Medicine and technology.
Given that life has been struggling to evolve and survive as long as possible through any means possible, reproduction is just a very poor form of immortality. If we could pass on our consciousness through our offspring akin to instinct then it would be by definition immortality.
Not only in Fate but in a lot of other fictions (like Hellsing), the anti-immortality perception is widespread. I believe that it is some authors' way of giving solace to themselves and to the human audience watching their fictions. Ultimately it's all a form of convincing oneself that one's own state is okay because there is no better alternative realistically. IF the immortality option is actually available to us, how many of us will honestly turn it down?!
As I wrote about this for Hellsing, I completely disagree with the "reject immortality for any reason" notion. Because it is anti-progress for attempting to put a stop on humans dreaming about and striving for the possibility to begin with. More of my view on Immortality

