Languages are constantly evolving and changing. Look at the vast difference that lies between a person who speaks Middle English and one who converses in the 21st century. Additionally, language changes over social groups and across space. Eventually, pronunciations change and evolve, while new words are added in the vocabulary. Over centuries, the change becomes huge and would eventually have a distinct variation from the mother tongue.
Why does the languages change?
Language change can happen due to various reasons. It could be due to contact with other languages, natural processes in usage and finally, social differentiation. When a child learns a language from his teachers or peers, he also hears the language been spoken by other people and using this knowledge he creates a language of his own. Sometimes the process of linguistic replication becomes imperfect.
When one language becomes dominant, it influences the other languages and eventually take over or get transformed. Change in language can also happen during trade and the people involved in trade would pick up one or two words in friendly exchange and eventually this catches on. If there are barriers like hills and mountains on different sides of civilization then language change may happen, but only very slowly. For example, people living in the highlands of Papua New Guinea speak dozens of languages, each of them vastly different from its neighbor. The people of Russia and Turkey and Iran situated on different sides of Caucasus Mountains speak different languages and so do the American Indians who live on the Pacific Coast.
An example of change in language
It is interesting to note how various languages and dialects were formed. Have a look at an interesting example of what the linguists call the Proto-Germanic.
This shows the relationship between Germanic languages. Though the words are not similar, they are related in some way or the other.