Depressed For No Reason?

Im only 13, 8th grade just started a little less than 3 months ago and im flunking this marking period, Im in my friends band as the b player, i worry about school, friends, family,piano,b guitar,and go to my friends house twice a week to practice. Im always depressed and in a bad mood. And recently i write depressing poems/ stories about life and death etc. Every time I write these stories/poems, i show that girl I like and she keeps them, she loves them and encourages me to keep writing them and to cheer up. When im happy she isnt, when im sad, she isnt I wrote the first one for a girl i like apologizing for being a jerk to her. Now i write them daily just about every night. I nearly cut today while hanging out with friends,got grounded, and my dad went to a teacher conference. WHY AM I SO DEPRESSED ALL OF THE TIME?!?!?! p.s. we hung out at a cemetery today (kinda weird)