On a different planet, humanoid creatures called immortals have achieved immortality.
The society is built specifically for these immortals, so in general, the immortals enjoy and cherish their lives.
For the sake of balance, however, some immortals must pass on or the number of resources on their planet will continuously dwindle.
The only way for immortals to pass on is if they go through a special ‘passing’ ritual, which has both biological and social implications.
(Edit) I've decided to elaborate a bit more on the background of the immortals:
The immortals are by all intents humanoid. They are still susceptible to disease, but disease is actually the source of their immortality. A long time ago, an extremely dangerous degenerative virus hit the entire immortal population. Only a small number of those with strong regenerative abilities survived, but the virus survived with them, generation by generation. The continuous destruction and reconstruction of the body of the immortal makes them unable to age.
As a result of their constant battle with the virus through history and their lifespans, immortals possess extreme regenerative abilities, and have completely lost their ability to feel pain. They have also, in essence, formed a mutual relation with the virus; both the lifespan of the immortal and the virus are inextricably tied.
Any new diseases the immortals come face to face with will cause suffering for some time, but the immortals can simply out-survive the disease while waiting for the cure to be developed.
Unfortunately, while the regenerative ability of the immortals is the source of their life, there is also a severe detriment: the need for energy.
Due to countless millennium spent fighting the virus, the consumption due to regeneration from the virus is relatively little. However, whenever an immortal suffers from a severe injury or disease, they will need to replenish their energy through food or other means. As such, they look down upon war, as this consumes a large amount of resources. All of these contribute to a large consumption of resources, and any planet the immortals settle on will eventually run out if the immortals are not cognizant of population control and efficient resource use.
Immortals are also not immune to being immoral (pardon the pun). When immortals commit a crime, they are imprisoned and denied energy, forcing the immortal and its virus to starve. This slowly degenerates the immortal until they are left in an extremely weakened state, called 'the withering'. If left to wither for long enough, the immortal will first fall into a coma, before eventually withering away, completely different from 'the passing'.
To add onto the point for interstellar travel, the immortals can settle new planets, but each new settlement journey is extremely perilous. They have infinite lifespans, and go into a state of hibernation during the journey, but suffer the risk of ‘withering’ on the journey due to limited supplies on each ship, and risks of something happening to the ship. As a result, it is very difficult and risky to settle new planets, so it reiterates the importance of keeping a balanced number of immortals on each inhabited planet.
Finally, 'the passing' is done by extracting the virus from the immortal, which causes the immortal to no longer regenerate, and allowing them to live out their final days before they pass to old age.
My question is, more specifically, what can the society do to help incentivize these immortals to eventually decide to pass willingly, and make the passing a joyous event, rather than a sombre one?