Next time you hold a door open for a stranger, consider this: Your simple action may jumpstart a beautiful friendship. It’s intuitive that doing a favor that evokes gratitude is likely to make the recipient more willing to do you a favor in return. There’s a cliché for that: You scratch my back, I scratch yours.It turns out, feeling grateful to someone who helps us can make us feel more connected to that helper. This can form the basis of a new relationship or strengthen an existing one.However, in an article for Psychology Today, Arash Emamsadeh points out that some people are hesitant to show gratitude. They may feel silly or embarrassed or fear the other person will be uncomfortable. Sometimes they think the other person already knows they’re grateful or think the reason for thanks isn’t important enough.

An Australian study by Lisa Williams and Monica Bartlett found that its participants who received a gratitude note were more likely to share their contact information for the purposes of continuing a relationship than those who received a simple acknowledgement note without gratitude. Receiving the thank-you note gave participants a sense of warmth and friendliness that led them to invest in trying to meet the unknown sender.

In a University of California Santa Barbara study, college sorority sisters were more likely to feel close to the older sisters who gave them gifts and who responded well to the little sisters’ appreciation. This closeness continued over time.

There’s a theory that animals, including humans, may perform favors for those who are not relatives in the hope that the favors will be returned down the road. Known as reciprocal altruism, the theory defines altruistic acts as those that involve effort or difficulty for the giver but do something good for the recipient. Studies have shown that chimpanzees also engage in this kind of reciprocity.

But whether you are the helpful altruist or the grateful recipient, the shared emotion is gratitude. This two-way beneficial relationship helps us identify the people who respond to our needs. We are then more likely to respond to their needs and thus to bond with them.