Baby is proud of building a skyscraper out of Mom's best pots, and Mom is terminally annoyed -- those circuits become confused and fail to strengthen. The key here is "repeatedly'':one dismissive harrumph will not scar a child for life. It's the pattern that counts, and it can be very powerful: in one of Stern's studies, a baby whose mother never matched her level of excitement became extremely passive, unable to feel excitement or joy
The trunk lines for the circuits controlling emotion are laid down before birth. Then parents take over.
Apparently, the brain uses the same pathways to generate an emotion as to respond to one. So if an emotion is reciprocated, the electrical and chemical signals that produced it are reinforced.
If a baby's squeal of delight at a puppy is met with a smile and hug, if her excitement at seeing a plane overhead is mirrored, circuits for these emotions are reinforced.