I'm not 100% sure the exact details on why you get sick, but for obvious reasons, alcohol is a poison. Too much poison in ones system results in vomiting.

Another reason is dehydration. Alcohol dries you're body out big time, that's why you get the doomed hangover in the morning. If you're body has lack of water, you'd get the same symptoms.

All in all, drink in moderation. They say it's healthy to drink a glass of red wine or a beer or two a day.