Development of Sense of Self
From birth to the first 18 months the infant’s identity becomes fused with caregiver and with maturity the infant individuates from caregiver to form his/her own identity. The infant needs to fuse and attach first before it can later individuate. Premature individuation, as during the state of insecure attachment, is perceived as stressful and as breaching social motivational need for love.
In accordance with Edmund T. Rolls’s theory of emotion, emotion is reflective of the state of satiation (whether achieved or frustrated) of expected reward. Emotional development starts at day one, because the expectation for fulfillment off primary and secondary (social) motivational needs start at birth. Satiation of need fosters a sense of perceived rewarding positive sense of well-being. Frustration of need or breach in reward expectation and outcome is perceived as aversive, stressful, and as warranting a defensive emotional response like angry aggression to mediate motivational goal attainment or sadness and depression to mediate motivational goal avoidance.
References
Rolls, E.T. (1999). The Brain and Emotion. New York: Oxford University Press. (Chapter 10-Reward, punishment, and emotion in brain design)