A vegetaran/vegan diet often lacks proteins and fats needed for your brain tovfunction properly. And also to help your body repair itself and keep you feeling okay.
A doctor who was a body builder said when he tried being a bodybuilder and a vegetarian for three years, that was the only time in his life he ever felt depressed and had low energy levels. he simply wasn’t getting enough proteins and fats to keep himself feeling good and healthy. Once he went back to a normal diet his energy levels bounced up and his depression went away.
Other people note it as well, it is well known these diets starve the brain and body of much needed fats and proteins.