Fifteen is the age in a boy's life where everything sucks. Period.
Your body turns against you in every way possible. Pimples, voice cracks, terrible body odor, unnecessary hair, constant horniness, and greasy skin and hair. Hormones run wild throughout your body producing an array of emotions you can't even understand let alone deal with.
Meanwhile, you have to deal with high school. Projects, homework, hard subjects, sadistic teachers, social crap to sort out, finding allies; they are very hard and common things there.
Also, all the girls you meet at school seem to flourishing into beautiful women. Their wonderful breasts jiggle and taunt you, although you can't have them. The mean ones make fun of you, the nice ones try to humor you, and if you find an absolutely great one that's willing to be your gal, good luck getting the money to pay for her. Businesses won't hire you for a year or so even if you did have the time to work a shitty job.
And there is no relief from this hell that is your life. Doing drugs or alcohol is a temporary release, if you can find, pay for, and not get caught with it. You're just barely smart enough not to commit suicide or run away from home. And you know that porn you're always watching will eventually get you in trouble. All you can do is wait it out.
Also, 15 is the fifteenth number from zero
Dude, why are you so down?