The encounter between a therapist, an infant, and his/her parents generates a variety of intense emotions, positive and negative. Some negative emotions or “negative feelings” are uncomfortable and make therapeutic work harder. An emotion felt as a “negative feeling” loses its value of being a communication signal and hinders the capacities of empathy and mentalization of the therapist. This implies that it no longer plays a role of being an incentive to act. The theory of intersubjectivity and the theory of attachment shed interesting light on these negative feelings and lead us to use them as a semiological tool, which allows a new understanding of some processes at work. The presentation of a first consultation with a mother and her 10-month-old infant illustrates how to use this perspective.