"There is one circumstance in which the extremity of do-gooders looks normal, and that is war. In wartime — or in a crisis so devastating that it resembles war, such as an earthquake or a hurricane — duty expands far beyond its peacetime boundaries… In wartime, the line between family and strangers grows faint, as the duty to one’s own enlarges to encompass all the people who are on the same side. It’s usually assumed that the reason do-gooders are so rare is that it’s human nature to care only for your own. There’s some truth to this, of course. But it’s also true that many people care only for their own because they believe it’s human nature to do so. When expectations change, as they do in wartime, behavior changes, too.
In war, what in ordinary times would be thought weirdly zealous becomes expected… People respond to this new moral regime in different ways: some suffer under the tension of moral extremity and long for the forgiving looseness of ordinary life; others feel it was the time when they were most vividly alive, in comparison with which the rest of life seems dull and lacking purpose.
In peacetime, selflessness can seem soft — a matter of too much empathy and too little self-respect. In war, selflessness looks like valor. In peacetime, a person who ignores all obligations, who isn’t civilized, who does exactly as he pleases — an artist who abandons duty for his art; even a criminal — can seem glamorous because he’s amoral and free. But in wartime, duty takes on the glamour of freedom, because duty becomes more exciting than ordinary liberty…
This is the difference between do-gooders and ordinary people: for do-gooders, it is always wartime. They always feel themselves responsible for strangers — they always feel that strangers, like compatriots in war, are their own people. They know that there are always those as urgently in need as the victims of battle, and they consider themselves conscripted by duty."
https://www.amazon.com/Strangers-Drowning-Impossible-Idealism-Drastic/dp/0143109782
"What do-gooders lack is not happiness but innocence. They lack that happy blindness that allows most people, most of the time, to shut their minds to what is unbearable. Do-gooders have forced themselves to know, and keep on knowing, that everything they do affects other people, and that sometimes (though not always) their joy is purchased with other people's joy. And, remembering that, they open themselves to a sense of unlimited, crushing responsibility."
Slightly longer, for some more context:
"An extreme sense of duty seems to many people to be a kind of disease – a masochistic need for self-punishment, perhaps, or a kind of depression that makes its sufferer feel unworthy of pleasure...In fact, some do-gooders are happy, some are not. The happy ones are happy for the same reasons anyone is happy – love, work, purpose. It is do-gooders’ unhappiness that is different – a reaction not only to humiliation and lack of love and the other usual stuff, but also to knowing that the world is filled with misery, and that most people do not really notice or care, and that, try as they might, they cannot do much about either of those things. What do-gooders lack is not happiness but innocence. They lack that happy blindness that allows most people, most of the time, to shut their minds to what is unbearable. Do-gooders have forced themselves to know, and keep on knowing, that everything they do affects other people, and that sometimes (though not always) their joy is purchased with other people’s joy. And, remembering that, they open themselves to a sense of unlimited, crushing responsibility."