From The New York Times:
Caryl Rusbult, a researcher at Vrije University in Amsterdam who died last January, called it the “Michelangelo effect,” referring to the manner in which close partners “sculpt” each other in ways that help each of them attain valued goals. Dr. Aron and Gary W. Lewandowski Jr., a professor at Monmouth University in New Jersey, have studied how individuals use a relationship to accumulate knowledge and experiences, a process called “self-expansion.” Research shows that the more self-expansion people experience from their partner, the more committed and satisfied they are in the relationship.
To measure this, Dr. Lewandowski developed a series of questions for couples: How much has being with your partner resulted in your learning new things? How much has knowing your partner made you a better person? (Take the full quiz measuring self-expansion.) While the notion of self-expansion may sound inherently self-serving, it can lead to stronger, more sustainable relationships, Dr. Lewandowski says. “If you’re seeking self-growth and obtain it from your partner, then that puts your partner in a pretty important position,” he explains. “And being able to help your partner’s self-expansion would be pretty pleasing to yourself.”