"Adam sinned! Why am i called a sinner?"
If that's the case, i might as well ask, "Why do i have my father's personality and my grandfather's nose?"
On a more serious note, Adam was the only one in all of God's creation who directly derived his life from God. God breathed on the body made of dust and then it became a living being.
So, Adam got the nature of God because he came directly from Him.
Now, on that basis, whoever comes from Adam directly would have the same nature he had.
When he sinned, he received the nature of sin and it passed on to the next generation and to the next and finally it reached me and you, which will be passed onto our next generation.
For the sake of argument, imagine if Cain and Abel was born before Adam sinned. Then, they woulld have received a nature without sin. Their descendents would have been different from that of their father, Adam, i.e without sin.
Now, apart from Adam, do you know anyone else who came directly from God? Yes, there was one. Jesus, as a son of man, was the second Adam to come directly from God.
Therefore, he received not a nature of sin but that of God.