Why does your head hurt so bad after a night of heavy drinking?

Dehydration.

Are you asking what causes a hangover?
If so, dehydration is one possible cause. Alcohol is a diuretic, which means it increases urination and flushes fluids from the body. So drink plenty of H20 if you’re hungover.
Here’s more information on hangovers.

One thing you can do to help yourself not have hangovers after a night of drinking is to have a beer or two (or whatever else you’re drinking) and then have a glass of water. Continue doing this and it typically helps you avoid hangovers. Of course this won’t really work if the plan is to get wasted as quickly as possible.

Eat something, preferably fatty and salty, before you go to bed. Excessive urination associated with drinking also means you’re depleting the salt levels in your body. The fat will neutralise the alcohol that’s still in your stomach.

chug a big glass of water and take a couple of aspirin before you crash after a long night of drinking. i learned this from an alcoholic roommate. doing this can make all the difference.