The idea that attachment is formed as a learned behaviour. This starts as we associate our caregiver (mother), the unconditioned stimulus, with the pleasure we get from food, an unconditioned resopnse / reinforcer. The caregiver is the secondary reinforcer as we see them linked to avoiding discomfort and gaining pleasure from the food. Eventually, we come to associate the mother with pleasure, and so an attachment forms.
Bowlby's Theory of Attachment
Bowlby's theory suggests that attachment is innate, as it is basically a means of survival and so the need to become attached has been born in us, similar to how animals imprint on the first moving thing they see (Lorenz's goslings). Bowlby's Theory also suggests that a main monotrpoic attachment should form within the sensitive period, or the infant will find it difficlut to form meaningful attachments in the future - the continuity hypothesis. Infants will go on to use this monotropy as a secure base from which to explore the world, and will use it to form thier internal working model, to judge future attachments based on the caregiver's reactions to the infant. Infants are also born with certain characteristics, known as social releasers, which make the caregiver want to look after the infant - smiling, crying, cooing, the 'baby face' (wide eyes, large forehead, squashed up nose).