That is a very good question and one many people may ask themselves.
Prominent veins on the hands and arms are due to variations in a person's genetics. The number one factor is the amount of subcutaneous fat. Less fat under your skin will cause the veins on your hands to be more prominent. More toned muscles in your arms correlate to prominent veins in your arms.
Dehydration causes veins to be less prominent.
I hope this answers your questions.