Imagine a world where life expectancy is fixed and the same for everyone at birth, yet people can engage in life expectancy "donation" – meaning that someone can extend your life three years longer at the expense of theirs. Talk about a likely scenario in this world that involves voluntary donation.
One voluntary donation involves donors and their loved ones. For example, children who wish to have more time with their parents could extend their parents' life expectancy at the expense of theirs. For loving spouses with different times of birth, the one who was born later could extend their partner's life so that they could pass away at the same time, without having to suffer the pain of losing the love of their life. If pets also have fixed life expectancy that is yet shorter than that of humans, the pet owners could also make such a donation willingly.