Why do I wakeup with a boner (my penis erect)?

Normal. Morning erection is a physiologic. All men without physiological erectile dysfunction experience this phenomenon, usually three to five times during the night. It typically happens during rem sleep. The cause is unkown. Full bladder might cause a reflex erection during sleeping.
Normal. The technical name is nocturnal penile tumescence. It is considered a normal event during sleep. Possibly related to growth hormone release during the middle of sleep . Lack of this condition is considered abnormal.
Because you're a man. This is normal especially when you're young. You're a grown man now and I'm surprised you didn't ask your friends or Dad -- they've had the same experience. Enjoy it.