Why are people stuborn?

Well, since I was 14 I have become more stubborn because I'm sick and tired of my friend being so bossy and stubborn to me and another reason was because all my life I got bossed round and I was so easy to boss round. I was getting tired of it. So as I said since I was 14, I started becoming stubborn and more stubborn each month and I'm proud to be stubborn so people don't have to boss me round anymore but who I'm most stubborn to is probably my mum