A recent study by Brent A. Mattingly, Gary W. Lewandowski Jr, and Kevin P. McIntyre examined how relationships can change each person in it, and the impact it has on the relationship itself.
The researchers asked couples to individually share how much they agree or disagree with the following statements:
“Individuals are more satisfied with and committed to their relationships when their self-concept has improved (either by the addition of positive content or the subtraction of negative content”
So if you made a positive change as a result of a relationship (like eating healthier or achieving a goal at work with your partner's encouragement) you were more likely to report you were happy.
The research also examined how self-change in relationships made participants more or less prone to infidelity.
Those who felt worse about themselves as a result of the relationship generally reported greater infidelity.
Adapted from: Glamour.com