Influenced by Darwin and increasing secularization, Sigmund Freud was the first psychologist to suggest there was an evolutionary chain reaction of human behavior of one generation to the next, in other words, that a parent's behavior affected their child's development. Some of the psychologists who accepted the ego, id, and superego framework of personality, (but didn't agree that all neurosis emerged from sexuality) developped a model called Attachment Theory. Their research showed how critical the bonding period was to the child's mental development. A bond between babies and the caregivers that was affectionate and predictable allowed what psychologists termed "basic trust" to develop, the foundation for their identity as an individual.
The child develops basic trust by moving safely through the phases of symbiosis to differentiation, from the soothing oceanic feeling of boundaryless oneness with mother, in Freud's terms, to learning that they have their own identity and can safely explore the world apart from her. The journey through the phases, as defined by Dr. Margaret Mahler provides the child with confidence to feel comfortable being apart from mother for increasing periods of time, and ultimately to develop their own unique identity as an adult.
A healthy and successful individuation process begins when the child learns that mother's ministrations and their own expulsion of tension through their bodily functions can relieve their unpleasant sensations. The child sees, furthermore, that when they make eye contact with their mother, that it is welcomed, and when they make vocal sounds or move their body to imitate their mother, that they are mirrored lovingly. Having their cries answered teaches the baby that their actions have an impact, and brings positive results that alleviate their discomfort; in other words, it teaches the baby that they have agency.
As individuality consolidates and the baby begins to walk and explore his environment, he needs to see his mother waiting for him or waving at him when he looks back at her. When he comes back to share what he discovered on one of his "adventures", dumping his findings in her lap, the subsequent cooing and interaction about what he found teaches the child that it's not only safe to explore, but healthy. Eventually, if the child had positive enough interactions with his caregivers, he learns that he can soothe himself in moments of crankiness- simply based on the fact that he's outlived them before- and feels more and more comfortable engaging with strangers and exploring new environments.