'Because I don't want to'
Anyone who has put that as an answer needs to take a long hard look at themselves.
You selfish selfish arseholes.
You could be carrying it and pass it onto someone who may be less healthy than you and therefore die as a result.
You ignorant, selfish people are the reason this thing will continue to spread and cause unnecessary deaths.
Selfish attitudes like that absolute disgust me.
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For humans to simply exist (unless they're off the grid, I suppose) is an inherently 'selfish' act, as you describe it.
They are continuously spreading germs (coronavirus and others) that directly cause death,, consuming products having toxic by-products that directly cause death, moving around in ways that directly cause accidents that cause death, burning fossil fuels that directly cause death, eating foods grown unsustainably that directly cause food shortages, causing death, and so on.
We could all do more to mitigate the damaging effects of our behaviour; wearing masks doesn't even scratch the surface.
Which is why this is hysterically detached from proportionality.