How do i get my parents to trust me?

My parents dont trust me. They wont let me hangout with my boyfriend or even my best friends who they know pretty well. I dont understand. I havent done anything for them not to trust me. Im 15 and i want to have fun. Im losing my friends because my parents wont let me hangout and be a teenager. Its not fair. Theyll tell me i can, then last min. say i cant. Does anyone have any advise on what to do?