Why does he do this?

Ok i have a boyfriend who i have fallin completly in love with, yet my dad is actin really up tight and he doesn't trust me, for example we all were in the house and he wouldn't let me stay in my grandma's room with her while he was in the living room.. he is not being fair, he makes me happy and my dad needs to understand that i am not alil girl anymore.. shortly i'll be 18..... why is he being so mean torwards me and not letting me be happy?