Languages change when speakers come into contact with new populations (think colonialism and trade), and they change when social groups adopt their own distinctive norms.
In some cases, languages change because the needs of the speakers change, particularly as new technology and ways of life require new ways to talk about them. In other cases, language changes simply because no two people are alike, and we are all, linguistically, products of the environment and people we interact with. Languages change when speakers come into contact with new populations (think colonialism and trade), and they change when social groups adopt their own distinctive norms.