Incorporating growth (i.e., broadening one's perspective of the self, partner, and world) is an important feature for the maintenance of satisfying and long-lasting intimate relationships. However, as relationships progress and people increasingly gain familiarity with their intimate partner, the opportunities for growth decline. According to the self-expansion model, one way that people can add growth to their relationship is by engaging in exciting (i.e., novel and arousing) couple activities. Although exciting couple activities have been associated with many relational benefits, it is unclear whether certain threat-related conditions might be related to poorer relational self-expansion and relational self-expansion outcomes, such as engagement in exciting couple activities. Through a 21-day daily diary study of community couples (N = 122), the present study investigated the effects of relationship threats on the occurrence of exciting couple activities and ratings of relational self-expansion. It was hypothesized that insecure attachment styles (i.e., trait-based) and daily relationship threats (i.e., state-based) would negatively predict people's overall exciting activity experiences and ratings of relational self-expansion. Data analyses were guided by the Actor-Partner Independence Model (APIM). Contrary to my hypotheses, there was inconsistent support for the association between attachment avoidance and attachment anxiety and relational self-expansion. Consistent with my hypotheses, people (and their partners) reported lower relational self-expansion, and also were significantly less likely to engage in exciting couple activities on days when relationship threats (e.g., rejection, disappointment, feeling misunderstood) were greater than usual. Furthermore, on days when people engaged in exciting couple activities and felt greater relationship threats, they reported a less successful exciting couple activity experience. Although there was evidence that relationship threats negatively predicted people's daily activity experiences on the same day, relationship threats were not related to the occurrence of exciting couple activities on the following day. In conclusion, this study offers evidence suggesting that state-based threat concerns might be one obstacle that couples face in promoting growth within intimate relationships.