Most Helpful Guy
I only get them when my blood flow is messed up, like when I'm in a poorly designed chair or when I wake up. I'd compare it to that feeling of tension your muscles get after a workout. That mixed with a wedgie. I notice that my boner is more sensitive when on purpose, which is kinda weird, because accidental boners are about as fun as your leg falling asleep.
Most Helpful Girl
At your age, mostly because that's when their testosterone levels start going through the roof. It's something new that starts happening to their body for the first time at puberty, so they have not yet gained a sense of control over it that will come with time/practice. I've heard from some guy friends, that around that time when they were younger they wanted to have sex with pretty much anything, they were that horny.
Yes, random boners do exist. Also remember that later on at least, boners do not guarantee the guy being turned on. A dick can get hard purely from stimulus sometimes, result in an awkward boner appearing at not so appropriate times.