As we all know, Levi is the person other than Mikasa who doesn't smile very often.
He usually has a emotionless facial expression.
It was also mentioned by Isayama Hajime that if Levi smiles, " He will die soon".
Fans started to freak out when Levi sort of smiled in chapter 69 of the manga ( Look at pic above to see ).
Let's just hope that beloved Levi isn't going to die anytime soon!