Experiences of closeness in romantic relationships are associated with heightened levels of relational well-being and mental health. However, individuals differ in the degree of closeness they desire in their relationships. This study used the construct of inclusion of other in self (IOS) to analyze discrepancies between individuals’ actual and ideal levels of closeness in their relationships and the implications of these discrepancies for relational well-being and mental health. A longitudinal survey of partnered individuals demonstrated that optimal levels of relational well-being and mental health existed when individuals had minimal discrepancies between actual and ideal IOS, regardless of their actual levels of IOS. Individuals whose actual levels of IOS moved closer to their ideal levels over a 2-year period reported improved relational well-being and mental health. Individuals with little to no discrepancies between actual and ideal IOS were also less likely to break-up with their partners over time.