There are many humans who are terrified of death, and who have gone to great lengths and horrific atrocities to overcome it; the sad fate of Kanto's Bulbasaur bears witness to Man's efforts to escape the inevitable. The gods never die, but those born mortal can never fully join them – even Bulbasaur bulbs only extend life, and all the wonders of medical technology struggle to combat the ravages of age – unless they receive the blessing of Xerneas.
Xerneas is an extremely compassionate deity, who always opposed my decision to draw a line of mortality between the divine and humanity. Given the choice, it would revive every corpse and abolish even the possibility of death outright – an idea which many philosophers and theologians have favored, but they have not seen the stagnation of a world without death. Xerneas was created to keep Yveltal's destruction in check, and the two have warred ever since that day so many million years ago, when most pokemon of the Precambrian were exterminated in a Yveltal-induced mass extinction and I hurriedly created Xerneas to prevent the extinction of life itself.
In time, Xerneas and Yveltal agreed on an uneasy truce, under whose terms Xerneas was forbidden to grant anyone eternal life. Yet a truce born of such a ferocious rivalry is often broken, and Xerneas has often taken advantage of any conflict with Yveltal to grant immortality to all living it deems worthy, and (if victorious) to undo every death suffered in their ferocious battles. Some have sought to pit Yveltal and Xerneas against one another, in the hopes that Xerneas would prove victorious and they could be rewarded with endless life; it is not wise to meddle in the conflicts of the Gods, however, and whole civilizations have been extinguished by Yveltal whenever Xerneas lost.