helper’s high
n. Feelings of euphoria and increased energy that occur while helping other people.
Older adults who live by the adage "it's more blessed to give than to receive" enjoy longer life spans than people still focused on "gimme" in later years, suggests a pioneering new study…

The study followed 423 couples for five years. All of the men were at least 65 years old. At the start, participants were asked if they'd given or received emotional or practical help in the past year. Five years later, those who said they'd helped others were half as likely to have died, says University of Michigan psychologist Stephanie Brown. Her study will appear in Psychological Science next year…

"Helper's high" is the term coined by Arizona State University psychologist Robert Cialdini to describe the euphoria reported by frequent givers in his research. These good feelings may lower the output of stress hormones, which improves cardiovascular health and strengthens the immune system.
—Marilyn Elias, “A generous spirit may yield generous life span,” USA Today, November 14, 2002
It's dubbed "the helper's high."

Researchers say the satisfaction people get from volunteering can cause a rush of euphoria, followed by a longer period of calm…This "feel-good sensation" should aid health by reducing stress with the release of endorphins, the body's natural painkillers.

The initial rush is followed by a longer-lasting period of improved emotional well being, which volunteers believed to be vital to health improvements they experienced.
—Carmela Fragomeni, “Today's fitness tip: Become a volunteer,” The Hamilton Spectator, April 24, 2002
1988 (earliest)
People who exercise vigorously often describe feeling high during a workout — and a sense of calmness and freedom from stress afterward. New evidence reveals that these same emotional and physical changes can be produced with activity requiring much less exertion — helping others.

An analysis of the experiences of more than 1,700 women who were involved regularly in helping others highlights these surprising effects. In many cases, this "helper's calm" was linked to relief from stress-related disorders such as headaches, voice loss and even pain accompanying lupus and multiple sclerosis…

Altruism has several advantages over exercise. Although the feel-good sensation is most intense when actually touching or listening to someone, it can apparently be recalled. Seventy percent of the magazine readers and 82 percent of the club members said their helper's highs would reoccur, though with less intensity, when they remembered helping.
—Allan Luks, “Helper's high: volunteering makes people feel good, physically and emotionally,” Psychology Today, October 01, 1988
Robert Cialdini didn't coin helper's high, as the USA Today article claims. The earliest use I can find for him is from 2001, but the phrase appeared back in 1988 in Psychology Today (see the earliest citation).