As adults, we sometimes struggle with calming our own emotions. But at least we know that we have the ability to do so.
Children on the other hand, often get totally overwhelmed by their emotions. If we can give them comfort and love when they are upset, they learn over time that after discomfort comes peaceful feelings and connection again. Over time, a connected child internalizes this cycle of emotional comfort after upset, meaning that they can find inner peace any time they look for it.
If we let them deal with their emotions by themselves, they are usually not able to get through their feelings and reach inner peace. Instead they can start acting out to get their emotional needs noticed in other ways. If this does not work, some give up on seeking connection and start suppressing their emotions to the point that they do not consciously feel much of anything. Since they are not used to receiving a compassionate response, they feel they have nowhere to turn. Over time this suppression disconnects them from others and they start to feel all alone in the world.
With the consistent loving care of an adult, this negative cycle can usually be easily broken with a younger child or a baby. But with older children, professional help may be needed to get back to connection and emotional health.