Some people think that saints must always be smiling, but actually, they have all arrays of emotion. Lots of the time saintly persons are grave and serious as they spread urgently important transcendental knowledge. When they are pleased, however, such a smile is uniquely attractive, as Srila Prabhupada explains in this short but sweet purport.
sa ugra-dhanvann iyad evababhase
asic ca tusnim aravinda-nabham
mukhena ceto lulubhe devahutyah
Sri Maitreya said: O great warrior Vidura, the sage Kardama said this much only and then became silent, thinking of his worshipable Lord Visnu, who has a lotus on His navel. As he silently smiled, his face captured the mind of Devahuti, who began to meditate upon the great sage.
It appears that Kardama Muni was fully absorbed in Krsna consciousness because as soon as he became silent, he at once began to think of Lord Visnu. That is the way of Krsna consciousness. Pure devotees are so absorbed in thought of Krsna that they have no other engagement; although they may seem to think or act otherwise, they are always thinking of Krsna. The smile of such a Krsna conscious person is so attractive that simply by smiling he wins so many admirers, disciples and followers.