The urge to compliment strangers is pretty universal. You see someone on the train with a band T-shirt you love: great choice! You clock that a dress has pockets: good find! There’s a particular joy in passing on these compliments; the delight on the stranger’s face is a gift you both receive.

But there is a line to be aware of—that which separates kindness from intrusion. Most of us err on the side of caution, for fear of being woefully inappropriate. A colleague once confessed to me: “Whenever I’m jogging and I see anyone jogging toward me I always want to high-five them and say they’re doing really well, but I never do.” She was afraid of being ignored, but she was also conscious of boundaries. 

Most of us appreciate a genuine compliment because it’s gratifying to fe...