September 24, 2015 5:34 AM EDT

The term do-gooder is often demeaning. It can mean someone who’s annoyingly earnest or self-righteous or an intrusive meddler. But even when it’s used to describe someone who does good deeds, there’s still hostility in it. Why? Why do moral people make us uneasy?

One reason may be guilt, or irritation: nobody likes to be reminded of his own selfishness or be told, even implicitly, what to do. But that’s not the whole story.

Ambivalence toward do-gooders also arises out of a deep uncertainty about how we ought to live. To most people, it’s obvious that we owe far more to family than to strangers. But do-gooders challenge that idea. If everyone’s family is loved by someone, they reason, why shouldn’t we help other families as we would help our own?

This raises myriad moral dilemmas, especially in extreme cases. One couple I interviewed adopted 20 children who needed a home, but they struggled when weighing the needs of still-unknown kids against the needs of the kids they already had. Another couple founded a leprosy colony in rural India. Their dogs were eaten by panthers; their two toddlers survived. But what if they hadn’t?

Everyone agrees that it’s good to help strangers sometimes. But when helping entails sacrifice not only from you but also from your family, figuring out the right thing to do gets much more complicated.

MacFarquhar is the author of Strangers Drowning: Grappling with Impossible Idealism, Drastic Choices, and the Overpowering Urge to Help

This appears in the October 05, 2015 issue of TIME.

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