The language variety that you speak as a native language is closely tied to your geographical identity. For instance, American English consists of words like 'squaw' and 'totem' - borrowings from Native American languages to describe the new natural environment that British settlers found when they arrived in America. These borrowings have since passed into general American English vocabulary.
It is also very much tied to your cultural and personal identity - in which case language becomes a touchstone for cultural heritage and identity. The language that you choose to use is often the language closest to your heart or that which you wish to be affiliated with. History will tell you this: William the Conqueror of France spoke French at his coronation in Britain, not English. The Norman French of Britain (this was long after William) started to speak a modified version of English to differentiate themselves from the Parisian French during the Hundred Years' War between France and Britain.
In many parts of the world today, people are increasingly adopting English as a first or second language for economic reasons. The trouble is, choosing not to speak your 'native' language can feel like a betrayal of your own cultural heritage and identity. Each language gives us the key to a whole new different world and unique viewpoint. A few years back, Malaysia retracted its English policy in schools and started using Bahasa Melayu as the language of instruction for science and mathematics, not least because of pressure from Malay nationalists who feared that the position of the national language was being threatened. This is a very real issue playing out across the world today - chances are, you don't have to look too far to see this happening around you.
This is just the tip of the iceberg - it's only what I can remember offhand - and I hope I've gone some way in answering your question! If you're interested, some related issues include: centrifugal and centripetal force in linguistics, the notion of a standard variety and linguistic diversity (linguistic human rights are a v interesting idea!).
