Life is about fitting in; as soon as you come into this world your life is shaped to fit the mould that your parents created. In many ways, adapting the way you speak and act in particular environments is natural.
Whenever someone is mixed with others, they will try to alter their personality to make communication easier: every 14 year old is going to behave differently when they are summoned to interact with adults, and every adult is going to change their demeanour (and language) in front of small children. It's perfectly acceptable.
Some people will enhance themselves in social situations, others will pretend to be someone completely different. The difference is: altering yourself in a way that you appear to be someone else is unnatural, and it is considered by many to be normal.
As a teenager, I can only comment on the way in which people my age behave around me, and even though I am pretty unobservant, I notice the change in personalities way too often.
I know that everyone changes when they are speaking to another group of people; they are perhaps louder or more shy; it's natural. However this is not the case when someone changes so often that the people around them can't differentiate between their personalities, let alone that person him/ herself.
Often it is easy to mix in with the wrong crowd, and to make friends with people who are worlds away from the real you; resulting in behavioural changes that can become permanent for months and even years. These can be good changes, but in a lot of circumstances the changes that occur are ones that completely alter that person into someone they aren't.
These negative changes are a problem because if you don't recognise them then you become a person you can't identify with, and you adapt to your new environment in a way that if it changes, or the people around you change, then you are left in a state where you can't recognise the real you. This is not a situation that you want to be in.
Or, the other issue with this is, that when you realise that you have altered your behaviour, you think that it is too late to revert into that person you understand, and to be with the friends you know are real and like you for you. Because, ultimately, being you is the most important thing you can be, even if you think that who you are will not be liked or treasured by others. Being you and being liked by people who won't take you for you, or want you to be someone else, is not important, and you should not bow down to those who believe different.
Being true to yourself is paramount in being happy; so don't unnaturally change yourself to become someone you can't recognise, because that will ultimately lead to regret, and also to fakery.
I hope that this post wasn't too deep, and that it may have helped someone who needed it.
See you soon!