What Causes Behavioural Disorder In A Child?

My 3 year old grand daughter often behaves in a rude manner towards people. She will often act as though she doesn't like you. example: Cameron sits down next to her and tries to engage in conversation. She hides her head and won't talk to him. Actually pushes him away. He eventually gets her to interact and play with her toys with him. The next day its the same thing all over again. This behavior is typical with anyone who comes into contact with her. Its as though every day you have to re-win her trust and affection as though you are a complete stranger. She also seems to have trouble with her politeness. When I or anyone for that matter gives her something she asked for and then asks her to say thank you she cries as though she's been scolded. She is not so good with please either. Its pretty much the same reaction. However, in preschool she uses her manners so I know she understands that these are good behaviors.