Individuals develop attachment through relationships with other people and pets, especially dogs, which can be associated with a person's mental health. These attachments can vary in strength in different relationships, for example, with a partner or a dog. Attachment to people is likely stronger due to the limitations dogs have in terms of behavior, verbal expression, and cognition. Nonetheless, attachment formed with a dog can have positive effects on mental health, such as reducing loneliness and increasing life satisfaction. Previous findings have been inconsistent; suggesting that attachment to a dog may contribute to mental health only when attachment is not established through relationships with people. The aim of this research is to examine differences in attachment to a dog and to various close individuals, as well as the moderating effect of the level of attachment to a close person on the relationship between attachment to a dog and loneliness or life satisfaction. Data were collected on a sample of 1199 participants aged 18 and over who have owned a dog for at least one year. The study used the Attachment Features and Functions Questionnaire, a single-item life satisfaction assessment, and the Short Form of the UCLA Loneliness Scale. Attachment functions were most prevalent in relationships with dogs, followed by partners, family members, and friends. Attachment to a dog was borderline insignificant (positively) associated to loneliness and was not associated with life satisfaction, and these associations were not moderated by attachment to a close person. It is possible that previous inconsistent findings can be explained by other factors, such as attachment style, which need further investigation.