Can teens develop depression for no obvious reason?

Yes: While one form of depression--reactive depression--is brought on by disturbing circumstances--many forms of depression come on without obvious source of anguish. A teen may only know he no longer ls like his old self and it feels awful. May be bored, restless, angry, sad, tearful, silent, sleeping, shouting, sullen, scared--or all of this by turns. You, a parent may be too. Be the adult. Get help.

Yes: But teen angst can be very powerful and look a lot like a depressive episode. Seek medical care it not remitting or if impacting function.