How can i be happy without bein sad?

well hears the thing im only 12 and 1 grandpa, 2 grandmas, 2 uncles, 3 cusont, 5 aunts, 1 realy close friend, and the best dog in the world have all died so now its only me my sis my dad and my mom. all my friends complain they have the worst life and i have no idea why they would even say that, not one of my friends know one person that died. well the point is i can never be happy because every time i start to be happy a voice comes in my head and says how can you be happy when you know they are dead, in fact befor i started to type this i was just curious but now as i type i am crying more and more. is there any way that i can be happy more than one minut. my dog was the last one to die, he was a golden retriver, he died last month and i had ever sence i was born, he ment the world to me and i never got to say good-bye. i need you to respond A.S.A.P cuz i cant stop cryin (how can i be happy without bein sad)