We are all social animals, which means that we need to be in groups and to be affectionate toward others to survive and thrive. It is simply nature that we long for touch, and love, which is so often shown through touch.
Babies are no different.
They also long to be touched and thrive when they are. When a mother or father plays with their baby, holds, talks to them, and cuddles with them, the babies smile and react positively to that stimuli. It is not very often that I see babies in strollers facing the same direction as their parents, so they can not make facial contact, with smiles on their faces. They are lacking the interaction they so desperately need to survive and thrive developmentally. Never do you see other primates leave their young, assuming that if they touch them too much they will become spoiled.