My little cousin would be a perfect partner in life, I don't understand why it's illegal.

I am a 17 year old guy from Turkey. In my country, it's legal to get married when you are 16 years old, but we are banned to marry our first cousins. My little cousin is my best friend forever. She is my best friend and I'd like to marry her. I love her more than anything. She is my most beautiful gift and I want her forever. I want to be by her side forever. I don't understand why the government is banning us to marry. I'd like to live happily with my little cousin. I don't understand why there is such an obstacle in front of me.