As kids grow up they grasp different kinds of mannerisms from around them. During growing years they also learn several shortcuts and tricks to get things done. One such trick is manipulation.
Upon denying a chocolate or a toy kids start throwing tantrums in public places and often parents cave in to the demands to save themselves from the embarrassment. Manipulative behaviour in kids stems from such incidents, mostly.
Once the kid starts analysing situations, it quickly understands that crying out loud or banging at things in public places or in front of outsiders will always lead to fulfillment of the demands.
Therefore, as much as it is important for parents to love their child, it is important to understand the behavioural changes in them in their growing years.