It s not so much psychological, it s vascular, and nicotine makes your arteries constrict.

When blood vessels constrict, the flow of blood is restricted or decreased, thus retaining body heat or increasing vascular resistance.

As the light is being moved from one eye to another, both eyes begin to dilate, but constrict again when light has reached the other eye.

This is because the left eye will not respond to external stimulus (afferent pathway) but can still receive neural signals from the brain (efferent pathway) to constrict.

If there is an afferent defect in the left eye, both pupils will dilate when the light is shining on the left eye, but both will constrict when it is shining on the right eye.