An infant is born with reflex responses. The (unconditioned) stimulus of food triggers the (unconditioned) response of pleasure. The person who provides the food becomes associated with the response of pleasure, therefore making the person the conditioned stimulus. The food giver then becomes a source of pleasure for the infant, regardless of whether or not the food is supplied.
When Hungry an infant experiences a drive state, and is uncomfortable. The drive motivates the infant to find some way of being more comfortable and reducing the drive state. Being fed satisfies the babies hunger, making the infant comfortable thus reducing the drive state. This is called drive reduction, which is rewarding for the baby and the child learns that food is the cause of this reduction, a reward or primary reinforcer. The mother or the food-giver is associated with the food, and becomes a secondary reinforcer. The infant will always seek this person because they are a source of a reward and the infant will become attached.