Blood volume has a lot to do with it. If you're well hydrated and you have good circulation, your veins will "stick out." If you're dehydrated, your veins go into hiding...just completely shrink up. Since I'm a nurse, I'm always looking at people's veins... you see these guys with these big huge veins and think how nice and easy it would be to stick them. LMAO.. we get warped minds after a while, what can I say.