Why cant i just be happy.?

i am never happy its my birthday to day and i am not really caring and i am never happy not in a bad way as if i am spoiled but everything i used to love or be happy about i am never happy about anymore i try to be happy but i cant and my dad and step-mum just think i am attention seeking but i am not i try to please them but it is hard to do that why cant i just be happy.