Often, cuddling alone will not quiet a newborn so adults assume that the infant is either wet or hungry. If the former proves incorrect, they place a nipple in the baby's mouth and the little tot experiences sensations of taste and of pressure against his or her lips.
That experience is very distracting because, in human beings, the lips are connected, by nerves, to a disproportionately large part of the brain which, in turn, is wired throughout much of the nervous system. Thus, oral stimuli distract babies from awareness of much that is unpleasant, ranging from aloneness to growing pains, so they try to put everything they can reach into their mouths: thumbs, paper clips, pennies, pins and even the buttons on the shirt of the people who are holding them.
Throughout life, humans seek oral gratification by engaging in a list of behaviors too long to mention. It is the second powerful drive which they acquire and it, too, greatly influences human behavior from birth until death.