On my left side on neck the vein pops out, why?

Normal: Neck veins distend when we lie with our head lower than our heart or when we strain. With straining, the pressure within the chest increases and the blood flow through the neck veins decrease resulting in them enlarging. This is normal. Levels of hydration, genetics, outside temperature and any increase in pressure within the chest will enlarge the neck veins. Your veins seem to be normal.L.