Since this is the case, we don't simply speak one dialect but many. To put it another way, dialects always exist along a speech continuum that we move back and forth across. The ability to adjust our speech to fix a particular social context is something we acquire as children. We learn not just to say things, but also how to say them and when and to whom. Language change is normal.
All languages change over time.
This is inevitable for a variety of reasons, and it is the reason we have different languages and dialects. Just because a language is changing does not necessarily mean it is becoming corrupted or is decaying. Every human language has been shaped by and changes to meet the needs of its speakers. While languages may differ, they are all complete languages. Our native language profoundly conditions our thought processes and shapes how we perceive and experience the world around us.