This is a quote from the Talmud, which, along with a similar quote from the Buddha are epigrams to a research report from Wake Forest University, the University of Nebraska and Washington University in St. Louis. Dustin Wood, Peter Harms, and Simine Vazire studied college students' attitudes toward others and their life satisfaction. They found that those who have the nice things to say about others have greater life satisfaction and lower levels of depression.
I'm not sure which comes first, the chicken or the egg. Are they kinder in their attitudes because they experience a sunnier life, or is their life sunnier because they approach others with a positive attitude? Or perhaps it's that those who tend to like others are liked in turn?