A person who has done someone a favor is more likely to do that person another favor than they would be if they had received a favor from that person. Similarly, one who harms another is more willing to harm them again than the victim is to retaliate.

This is an unbelievable finding. In social situations, you can hack this by making someone do something small for you, then asking for your true favor. It’s such a small favor that they will say yes, and due to cognitive dissonance their brain will rationalize that they must like you enough to do you a favor in the first place. This is also called the foot-in-the-door effect.