why do teenage guys find it so hard to gain muscle? (problem fixed) 

Good video bro. I've started eating more meat: Chicken, Tuna, Lamb and Kangaroo, And I've researched on this aspect. Carnivore diet is okay for a short term because your body requires fibre and removing the fibre from the diet can cause Appendicitis. So I consume fibrous foods with a meat rich diet to maximise gains and overall health. Our ancestors ate complex carbohydrates along with meat everyday. And we must not forget that.