Bruce Perry notes that belonging and being loved are a central part of being human. We are a social species that are meant to live together in community, connected to others emotionally and physically. Much of the functioning of the brain and body is designed to help us manage and maintain social interactions. The capacity to relate in meaningful and healthy ways is shaped by our early relationships.
Repetition of an action leads to change in the brain. However, specificity is also important in neuroplasticity. To change a particular part of the brain, that part of brain must be activated. This applies to learning how to play a sport or a musical instrument, but also to the ability to love. If you have never been loved (or did not receive enough) the parts of the brain involved in loving will be un-developed.
With use and practice, these capacities can begin to emerge. When the unloved are loved, they can become loving.