Both clinical observations and empirical data suggest that metacognitive functioning is a factor strongly associated with a good psychotherapeutic outcome. It has been suggested that some interpersonal social motivations (i.e., attachment and cooperation) may be associated with different levels of metacognitive functioning also within the therapeutic relationship. The aim of this study was to explore the relationship between cooperation and metacognitive monitoring within 58 psychotherapy sessions from seven different patients. All patients were initially assessed through a detailed psychiatric interview. Patients' adult attachment styles were evaluated using the Attachment Style Questionnaire. The association between the activation of patients' interpersonal social motivations (e.g., cooperation and attachment) and the modifications of metacognitive abilities during sessions was investigated using the Assessing Interpersonal Motivations in Transcripts method and the Metacognition Assessment Scale have been used. Our results showed that the activation of the patient's cooperative system is positively associated with an increase in metacognitive functioning, while the activation of attachment is not. The results of the present study have important implications for clinicians: they give empirical support for the role of cooperation in fostering metacognition within the therapeutic relationship.