Small liquid drops assume spherical shape because:

1. Of adhesion
2. Of the pressure from all sides
3. Of the gravitational force
4. The liquid tends to have minimum surface area due to surface tension                                                                                                                                                                                                                                
   

The spherical shape is because drop always tends to acquire minimum surface area due to the surface tension, and for a given volume, the surface area of a sphere is minimum.