When it is a need there is a sense that something is missing like you are deprived of it. In this case, the person thinks there he/she is incomplete and only that need will fill that void in order to become whole. If the need is not fulfilled the person will be in a continuous sense of dissatisfaction for that need is seen as his/her path to happiness. Yet, many times even if that need is supposedly satisfied the person still senses the missing part. This is because the need is an illusion of fulfillment, a future promise of happiness. The reality is the person is already complete. The sense that something has to be filled comes from one’s own perception that they are incomplete. If the person is unhappy then identify why, the reason behind it and deal with it instead of assuming something will make it go away. It is a form of escapism yet the object sought will not be found because it is not the answer.
On the other hand, wanting something is on a different level. In this case the person does not feel incomplete yet he/she seeks something in particular not for the sake of happiness rather as a natural human behavior. A person may be motivated to attain something for a particular purpose but it is not to feel whole.