When I was a student, there were days when I would wake up feeling down and emo and depressed for no apparent reason. I'd be quiet and sullen in class, keep to myself and answer questions only when I have to, or I'd simply skip class and sit by the lake or walk around the city until I feel better.
I still get days like that.
Only that now, skipping class is not really an option.
Rather, I'd (have to) plaster a big smile on my face, muster up every ounce of enthusiasm left in my system and try to brighten up the days of students feeling down and emo and depressed instead.
It just drains you and leaves you so very, very exhausted.