I think it's attractive because it's masculine but why do males have bulging veins?

Body fat and muscles: The biggest factor on the visibility of veins on arms is the amount of subcutaneous [ in the skin] fat that you have. Also the larger and more toned the muscles are the more visible the veins. Men tend to have lower body fat and larger muscles.

Prob Not Varicose: The bulging veins you speak of are probably dilated superficial veins - not varicose veins. They are usually seen on the arms, but can be seen on the legs as well. Varicose veins are a different entity.

Here's why: A man with prominent veins very likely has little body fat and also has more blood on board -- it's stored in the veins of the body. Some of this is genetic, and some reflects fitness training. An aerobic athlete is likely to have 20-30% more blood volume, which explains why track athletes tend to have veiny arms. Heavy muscles on the thoracic inlet may also make arm veins prominent.