Why do we have different languages?
Different languages were created over a lengthy period of time due to different colonies separating and cultures gradually morphing. Each colony made its own way of communicating - often more individual than the others - to show pride and uniqueness in their own people.
For instance, when referring to Germanic languages and how they made their way to England, we see how the language changed constantly due to environmental factors. The Saxons, speakers of Saxon, a Germanic language, were invaded by the Normans who introduced Norman French to the language of Saxon. The Vikings also had an effect in the north of England as they spoke Norse as well as the Romans, who invaded some time before and introduced Latin, thus the English language we speak today was born and it will continue to forever change.
Dialects also cause the language to change. As mentioned above, some colonies of people wished for their language to be unique and individual from another. The language of Swiss-German is a good example when comparing it to German. Although, some time ago, these were the same people, they purposely created a language - which did have some environmental effects due to bordering with France and Italy - in order to appear different from the common Germanic man.
Standard German (Deutsch): Und ich habe Heimweh nach den Bergen
Swiss-German (Schwyzerdütsch): U i ha Heiweh nach de Bärge
And finally, due to colonies of the world not working together. As humans, we've forced ourselves away from one another for thousands of years, thus we have developed different ideologies, thoughts, cultures and languages from other people. It's pretty simple if you think about it in that sense.