Simple Summary Past research regarding the impact of companion animals on well-being has yielded variable results, with some studies finding that companion animals have a positive impact on mental well-being and others finding neutral or negative impacts. This study explored potential causes for these contradictory results, measuring the relationship science concepts of attachment, self-expansion, perceived responsiveness, and perceived insensitivity within the human-companion animal relationship, as well as mental health outcomes of depression, anxiety, and affect; and loneliness as a mediator between the relationship science concepts and the mental health outcomes. Attachment, self-expansion, and perceived pet insensitivity all significantly predicted at least one mental health outcome. We also found that loneliness mediates the relationship between attachment, self-expansion, and perceived pet insensitivity, and all of the mental health outcomes. Our results indicate that these relationship dimensions play a role in the well-being benefits derived from having a companion animal.Abstract Research into the impact of companion animals on well-being has been both extensive and inconclusive, with studies finding both positive and negative relationships. The present research explored three previously unexamined relationship science concepts that may help clarify whether companion animals provide well-being benefits: self-expansion (the process of adding positive content to the self through incorporating new resources and perspectives into one's identity or engaging in novel, exciting activities), perceived pet responsiveness, and perceived pet insensitivity; as well as attachment. We focused on dog and cat owners' depression, anxiety, positive and negative affect, and loneliness through an online survey with a large sample population (N = 1359). We found that perceived pet insensitivity is a significant positive predictor of depression, anxiety, negative affect, and loneliness; that attachment is a significant positive predictor of depression, anxiety, and loneliness, and a significant negative predictor of positive affect; and that self-expansion is a significant positive predictor of positive affect, and a significant negative predictor of loneliness. Loneliness emerged as a mediator in the relationship between perceived pet insensitivity, attachment, self-expansion, and all mental well-being outcome variables. These findings indicate that perceived pet insensitivity, attachment, and self-expansion may play an important yet neglected role in well-being outcomes.