In a Guide Dog population, there is a higher return rate of handlers’ second dog compared with any prior or subsequent dog pairings. This phenomenon is called Second Dog Syndrome (SDS). Qualitative studies have identified defining features of this syndrome in Guide Dog handlers; however, there has been no peer-reviewed study into the experience of SDS in companion-dog owners or handlers of other assistance dogs. As such, this exploratory study aimed to define the SDS experience in companion-dog owners and assistance-dog handlers. Ten participants, all women, were recruited to participate in focus groups for companion-dog owners (n = 5) and assistance-dog handlers (n = 5). Transcripts were analyzed using thematic analysis. SDS was characterized by a strong bond with the previous dog, ongoing bereavement related to the previous dog loss, negative emotions related to the successor dog, and inability to bond with them. This appeared to be due to comparisons made between the successor and previous dog and unmet expectations of the new dog. For companion-dog owners it was also characterized by a fear of getting hurt again; for assistance-dog owners it was an inability to trust the successor dog, differences in work ability, and a threat to their independence. Time since the loss of the previous dog, awareness of the phenomenon of SDS, and support from the community also influenced the SDS experience. One unexpected finding was that SDS was not exclusively linked to second-paired dogs and that it may be more aptly named “Successor Dog Syndrome.” Given that poor outcomes associated with unsuccessful dog–owner bonding may result in relinquishment, this study provides an important first step to being able to quantify the experience of SDS to develop interventions in the future.