Dollard and Miller's (1950) secondary drive hypothesis suggests that, through classical conditioning, the presence of the mother becomes a source of pleasure in itself for babies:
Hunger and thirst are primary drives as they are innate - satisfying these needs is essential for survival. Relief from hunger is a primary reinforcer. Because mothers feed babies an estimated 2000 times during their first year (Dollard & Miller, 1950), the mother becomes a conditioned stimulus whose presence produces a conditioned response of pleasure.
Sources of pleasure, by nature, are reinforcing, therefore the mother becomes a secondary reinforcer. Wanting the mother to be near is a secondary drive – secondary because it has been acquired through classical conditioning.
Learning theory can also explain the mother's bonding with the child; the mother receives reinforcements and punishments for behaviours that affect the baby's well-being. For example, feeding a hungry baby, or changing a dirty nappy will mean that the baby stops crying (negative reinforcement) and the babies will smile and make cooing noises when caregivers talk to them (positive reinforcement).
Reinforcements can also come from signs of the child's happiness and development, for example, the mother may be excited by the baby achieving milestones like sitting up, crawling, babbling, first words etc.
Additionally, reinforcers that have been acquired due to sex-typing during their own childhood can increase caretaking behaviours (Gewirtz, 1961). For example, girls are reinforced by adults for looking after dolls, so looking after a baby may itself become reinforcing.