Margaret Pennoyer, an elementary school teacher in Manhattan, had just returned from a bachelorette party in Napa Valley when she received an email that had been sent to all the guests. The two organizers had itemized each woman’s individual expenses, which they had covered, and requested reimbursement through Venmo, an app that transfers money between users who have linked their bank accounts to their phones. Ms. Pennoyer owed $31.98 to one woman and $20.62 to the other.

In a previous time, the organizers likely would have asked everyone to bring enough cash to repay them in person or to mail a check afterward, courteously rounding down to $30 and $20. But the Venmo request, calculated to the penny, struck Ms. Pennoyer, 29, as emblematic of how the app, the most popular among her fellow millennials for everything from entertainment expenses to rent shares, “changes friendships and makes them more transactional,” she said. “It’s nickel-and-diming everything, literally.”

By rendering payments between friends nearly invisible — no cash changes hands, no checks are written — Venmo theoretically should make these relationships less obviously transactional. Yet not only does it encourage pettiness, distilling the messiness of human experience down to a digitally precise data point, but by making it so easy to pay someone back for purchases as trifling as a coffee, the app arguably promotes the libertarian, every-user-for-himself ethos of Silicon Valley.

“It’s making people less generous and chivalrous,” Ms. Pennoyer said. “It used to be you’d go to a restaurant, and you’d put down your credit cards and split it 50-50, even if one person had steak and one had chicken. But now people pay exactly to the cent.”