Recent circumstances with a adult loved one caused me to need to understand this.
Adopted children often suffer from this, through no fault of their own. Those early years of nurture form the basis of learning and trusting in human interaction for that child for the rest of their lives, all the way into adulthood. If there are no nuturing bonds, or those bonds are in turmoil, through either abandonment, constant change in primary care givers, or through abuse, in the first year or so, it is devastating to that future child, young adult, and later adult.
Some times, nothing can correct that inability to be able to have loving relationships with others, as there simply is no inherent trust, no matter what one does, or how greatly one pours love into that heart.
Sad, really
But be really careful what sources you read. “Reactive attachment disorder” is a real diagnosis, but it’s also a favorite of dangerous quacks who promote dangerous and damaging “therapies” to parents. The child deaths from “forced water drinking therapy” and “rebirthing therapy” were due to the activities of these self-styled “therapists”. Some have also advocated physically forcing adolescents to submit to being cradled in an adult’s lap like babies while being fed from a bottle, and aggressive punishment for failing to maintain continuous “loving” eye contact at all time when a parent is speaking to them.