Similar to what @chazmaz said. There are lots of reasons why a person might not be able to emotionally attach, aside from a normal lack of “interest”. Assuming a person isn’t disinterested in attaching for “normal” reasons, such as lack of emotional or physical attraction, but is in fact emotionally or physically attracted to someone else, but still unable to form a lasting and healthy attachment to that person, a common reason is learned avoidance resulting from harmful experiences emotionally attaching in the past. A commonplace scenario involves the child of divorce. During certain ages, particularly around 7 years and up, boys and girls that observe unhealthy conflict or abandonment between spouses can learn that forming emotional bonds with people is dangerous and maybe not really “worth it”. Later on, these individuals may have difficulty trusting, being vulnerable, taking chances, and expressing emotions towards others they might otherwise be attracted to, because they haven’t resolved the losses, or healed from, and reframed the losses earlier in life, in ways that allow them to invest their emotions in new relationships. For many in this position, it’s easier to have lots of shallower relationships, where they don’t run the risk of being emotionally hurt again.