Christians should be smiling they have the joy of the lord.
If you mean God, then all religion followers who believe in this entity must be brought in. For verily, the Bible says God loves everyone, even sinners.
As far as that alone being the reason to smile, there are times when and where life is difficult, for a variety of reasons and the pain brought on, hinders a smile from forming. It doesn't mean that a person is not grateful for their life, just that sadness has stepped in.