If, say, 50% of people who die do so from extreme old age or wasting away from a disease they'll never recover from, what's surprising about a culture where 10%

If, say, 50% of people who die do so from extreme old age or wasting away from a disease they'll never recover from, what's surprising about a culture where 10% of that population decides to end their life a few months early on their own terms? Stepping back, why is something outside your own cultural norms unbelievable in a general sense instead of it feeling natural to you for there to be different cultures? You don't have to agree with another culture, but why is it uncomfortable for it to exist?