Why is my brother such a wuss?

I mean seriously. He makes a huge fuss over absolutely nothing. I accidentally got the teeniest bit of blood on his bed and he freaks out like it's a biohazard. He refuses to go in his room until someone takes away the sheets and sterilizes them. I mean, come on, he's 14. And we're twins, so it's not like he's never seen my blood before. Why is my brother such a baby?