Why is it that i am extremely gullible?

I am extremely gullible people tell me to do something and i would immediately do it. I am also unaware that people tease me like i think that people are being nice to me but instead they are just mean and i just don't know that they are mean to me. People tell me to do something and i would do it and i was wondering how can i get smarter about not giving into what people tell me to do something?