Veins constrict to keep you warm (counter-current heat exchange etc) and draw closer to the surface to cool you down.
Having veins that show are the product of low bf% (veins don't show through much fat) and a hot body temperature (eg. After aerobic exercise they surface to cool the blood).
They also expand to provide glucose to working muscles more quickly, or because blood flow is occulted by working muscles (like a tourniquet)
I have never heard of people growing new veins(?)