Close relationships (with spouses, family, friends, community members) are the biggest factor keeping people happy throughout their lives, researchers discovered. People with strong relationships are happier, and physically and mentally healthier, than those who are less well-connected. (The researchers are still studying the connection between relationships and physical health -- there's evidence that good relationships result in lower levels of stress hormones, and less chronic inflammation.)

Other ingredients for a long and happy life include not smoking or abusing alcohol, exercising regularly and finding work-life balance, the Harvard study found. "Rather than just being your grandmother's good advice, there's real science behind this," Waldinger says. "You can quantify the number of years you'll live longer, if you do these things."

Find ways to perform small, random acts of kindness during your day. These acts can be incredibly simple, from complimenting a stranger at the grocery store on his or her shirt to making your spouse coffee before work to engaging a co-worker you don't usually talk with in a friendly Zoom chat.