Responses
I totally thought that too!!!!! Like he misses the smell and/or taste
We never see Mo smoke in any illustration, future or present. So this is the first time and probably the only time he ever smokes.
He didn't smoke before and he doesn't look like he knows how to smoke given that he coughs:
http://www.mangago.zone/read-manga/19_days/bt/manga/19_days/c224/pg-4/
My conclusion? He smokes because he misses his bf He Tian on Christmas Day and none of you can change my mind.