Children suck on things because sucking is one of a baby’s natural reflexes. As infants get older sucking serves many purposes. It may make them feel secure and happy and helps them learn about their world to suck on their thumbs, fingers, pacifiers or other objects. Young children may also suck to soothe themselves. Since thumbsucking is relaxing, it may help induce sleep.
Thumbsucking, in an adult, often manifests habit, as opposed to a mental problem or the like. According to the voluminous data on thumbsuckingadults.com,
“Many adults engage in this behavior as a reaction to stimulus in their environment such as sleep, reading, watching television and, yes, stress. They’ve learned to respond to this stimulus by sucking their thumb, a pattern started, typically during infancy, or even prior to birth. The reason why its popularity isn’t well known is due to social pressures to keep the behavior stealthy since it’s often inaccurately associated with immaturity.”