It's not explicitly stated, but I get the impression that Culture citizens are generally very pro-social and willing to sacrifice for others' benefits and may w

It's not explicitly stated, but I get the impression that Culture citizens are generally very pro-social and willing to sacrifice for others' benefits and may well have been engineered with that aim. They were willing to sacrifice nearly a trillion lives in the Idiran war, which was voted on by the entire population. Even after that vote, it was very easy to opt-out and I get the sense that most of those who did were motivated by genuine pacifism rather than personal fear. The most dangerous and uncomfortable jobs like Special Circumstances and Contact seem to have an enormous surplus of volunteers and it's even hinted that there might not be any need for humans to do most of those jobs; they exist just because humans want to do them.