The problem is, what do you define as changing yourself to fit with other people? There's being weird and there's being weird, and the latter isn't a good thing. Like Bent said, picking your nose, it may be sanitary but it's against social etiquette and we live in a society, so we have to respect these rules. If you don't respect these rules, then you'll make other people feel uncomfortable and they will dislike you for it.
For example, what if you didn't like washing? You don't care what others think, so why should you wash? Because you would stink, and other people don't like that. Have you ever sat next to someone on a bus who doesn't wash properly, it's not nice.
Part of caring what others think is being aware of what others feel. You have to know when to shut up, when to stop calling someone, when to back away, when to speak up and when not to etc. You may not want to, you may be loud person but sometimes you just have to keep quiet or you'll be seen as annoying, obtrusive and inconsiderate.
We live in a society, with other people, and we can't just expect everyone to accept us exactly as we are. How would you work alongside someone who expected that. We have to be attentive to what other people think and feel, and can't just expect them to accept us completely as we are. I don't mean completely change who you are, clothing, music/film tastes or whatever, but still subtly changing parts of your personality to fit in with who you're with.
For example, if you're with a loud group of people, you're loud but if you're with a group of people who are quite quiet, you'd be quiet too otherwise you'd make them feel awkward.
I admit that in the case of your closest friends or a partner they have to be able to accept you for who you are because these are the people who love and care for you despite your flaws. But in the general world, you can't expect people to just accept you, it's not how life works.
And tbh, even if you say you have lots of friends and are exactly who you are amongst them, well, you're slightly disillusioned. You're not you the whole time, otherwise you wouldn't be able to have friends in different social circles because they would have different ways/atmospheres/whatever. It's just little things, but you will change slightly.
Sorry if this doesn't make sense, it's a bit of a ramble xD