Smile when the mouth is crooked under normal circumstances for both sides of the lip muscle strength asymmetry, disease conditions may be facial paralysis and other diseases caused by the following. 1. Under normal circumstances, the upper group of lip muscles, including the smiling muscle, zygomaticus major muscle, and levator ani muscle, can pull the corners of the mouth upward to show the smile. Under normal physiological conditions, there may be asymmetry in the strength of the muscles on both sides, causing the mouth to look crooked when the smile is displayed. 2. Diseases: central facial nerve palsy shows that the opposite side of the mouth can not be lifted, peripheral facial nerve palsy shows that the affected side of the mouth drooped, the healthy side of the mouth upward crooked, all of the above can lead to smile when the mouth is crooked. There may be other reasons why your mouth is crooked when you smile, so we suggest that you go to the hospital in time for early treatment.