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[Stranger]

That is why self-sacrifice (the giving up of one's own interests or wishes in order to help others or to advance a cause.), if done to your own detriment, is an unbalanced action.

I would qualify that by saying that we are naturally egotistic, predisposed to value our own well-being more than that of others.

Therefore lots of actions that are in fact closer to a self-other balance will feel like sacrifice to us. Learning that kind of "sacrifice" - placing others' interests on a more equal footing with our own - is a core part of the STO vs STS choice.