When a baby becomes stressed, their body produces and secretes the stress hormone cortisol.  When a baby is comforted, cortisol is reabsorbed if that comfort happens soon enough, and the stress response is immediately reduced. 

If a baby is not comforted enough, there are high levels of cortisol continuously.  This has a neurotoxic effect – in other words, brain cells are actually destroyed by this circulating high level of stress hormone.  High levels of cortisol also suppress empathy centres in the brain.  Therefore a child who is constantly stressed and not comforted find it difficult to be empathetic with others.