A few paragraphs from The Neuroscience of Psychotherapy:
Cortical inhibition and descending control are also central to affect regulation. The rapidly changing and overwhelming emotions displayed by every young children reflect this lack of control. As the middle portions of the frontal cortex expand and extend their fibers down into the limbic system and brainstem, children gradually gain increasing capacity to regulate their emotions and find ways to gain soothing, first through others, and eventually by themselves. ...
Although both the left and right cerebral hemispheres are developing at very high rate during the early years of life, the right hemisphere appears to have a relatively higher rate of activity and growth during the earliest years (Chiron et al., 1997). During this time, vital learning in the areas of attachment, emotional regulation, and self-esteem are organized in neural networks biased toward the right hemisphere. Somewhere around age 3, this pattern of asymmetrical growth shifts to the left hemisphere.
The maturation and sculpting of so much of the cortex after birth allows for highly specific environmental adaptations. The caretaker relationship is the primary means by which physical and cultural environments are translated to infants. It is within the context of these close relationships that networks dedicated to feelings of safety and danger, attachment, and the core sense of self are shaped. The first few years of life appear to be a particularly sensitive period for the formation of these networks. It may be precisely because there is so much neural growth and organization during sensitive periods that early interpersonal experiences may be far more influential than are those occurring later. The fact that they are preconscious and nonverbal makes them difficult to discover and more resistant to change. Because these neural networks are sculpted during early interactions, we emerge into self-awareness preprogrammed by unconsciously organized hidden layers of neural processing. The structure of these neural networks organizes core structures of our experience of self. --- pp. 70-72, The Neuroscience of Psychotherapy