Self-expansion theory suggests that we are more fulfilled in our relationships to the extent that our partners provide us with opportunities for growth. New relationships are often intensely passionate because there is so much to learn about a new partner (opportunities for self-expansion are high). Over time, however, self-expansion slows as we get to know our partners. This is partly why for many couples passion fades as the relationship becomes more established. According to self-expansion theory, one way to maintain a more satisfying relationship is to engage in novel, exciting activities with your long-term partner. Science of Relationships - | - Do People Have More Sex on Vacation?