In the habit of every child or actions, there is always an explanation, and so with thumb sucking. Thumb sucking is a natural step that children suffer. There are some cases where children do not start sucking their thumb.
Some start with other parts of their body – their palm or any part of the hands until they become more coordinated and focus on the thumb or other body parts in their courses. Implementing effective procedure can be helpful but you can also take the help of some kind of products such as The Hand Stopper Thumb Guard at https://www.amazon.co.uk/Age-2-7-Stop-Thumb-Sucking/dp/B00VQU2DZS.
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The habit of thumb sucking, like how some experts believe, could start even during the pre-birth. That is, when the babies are still in the womb and completing the age of gestation. However, it is not yet certain. What is certain is that the sucking reflex is first associated with means on how children eat, either through breastfeeding or bottle feeding.
Apart from this, and which would also explain why babies who do not rely on breastfeeding or bottle-feeding to eat more to continue the habit of sucking, sucking is a natural way for babies to calm down. So it is like therapy for them. Whenever they feel sad, agitated or upset, they find relief when they suck their thumb or finger or a favorite body part they have chosen.
That being said, you see the importance of careful procedures to help your child stop thumb sucking. A negative or aggressive approach would just make your child more cling to his habit of thumb sucking. His instincts told him he is in danger or is in a situation that does not feel good. And therefore his natural reaction would be felt at ease, so more thumb sucking.