There are a number of theories concerning language acquisition. B.F. Skinner, the famous behavioral scientist, postulates that language acquisition is dependant on environmental factors, especially how often sounds are correlated not only to objects, but also to actions and events.
However, numerous critics have problems with Skinner's theory. These critics argue that the speed at which language is acquired and the complexity of language cannot be totally dependent on imitation.
Noam Chomsky, the noted linguist, believes that our brains are "wired" for language acquisition and actually have a "Language Acquisition Device" or LAD.