Chris Kline doesn't like to tell his wife of 17 years, Tara, that he loves her. He prefers to show her—by loading her favorite songs on her phone and warming up her car on cold mornings. While she was away on business recently, he surprised her by painting her home office in her favorite colors, Mardi Gras purple and gold.

"Saying 'I love you' is just words," says Mr. Kline, a 42-year-old engineer from Shoemakersville, Pa. "I like to do things that require effort, planning and a little bit of sacrifice. It shows you are putting the other person first."

Researchers call this "compassionate love"—recognizing a partner's needs and concerns and putting them ahead of your own. "It's not just making people feel good," says Harry T. Reis, a University of Rochester professor of psychology. "It's a way of communicating to the other person that you understand what they are all about and that you appreciate and care for them."

Since 2009, Dr. Reis has been studying 175 newlywed couples from around the U.S., asking how they show their spouses compassion. His findings, not yet published, indicate that people who discover ways to regularly show their spouses this kind of love are happier in their marriages.

Small selfless acts between spouses aren't just nice—they also are necessary, experts say. When acts of kindness and caregiving disappear, it is an indication the relationship needs help. And when you are compassionate to your partner, it increases your own happiness at least as much as the other person's.