At the time, that one more drink seems like a good idea. Alcohol causes the effects of the brains neurotransmitter GABA to increase. Neurotransmitters are substances that chemically connect the signals from one nerve to the next allowing a signal to flow along a neural pathway. GABA is an inhibitory neurotransmitter that acts on motor control, vision and and anxiety. Its actions result in relaxation and sleep. So when you drink alcohol, your brain functions are depressed, making you happy and a bit wobbly. Hopefully you don't drink so much that you pass out.
Then you wake up the next morning feeling like a grisly bear is using your brain as a trampoline and the taste in your mouth makes you think that a badger might well have climbed in during the night and died. Why would anyone deliberately do this to themselves?
One of the main culprits in a hangover is dehydration. Ethanol (alcohol) is a diuretic, which means that it causes us to lose water. It acts on the pituitary gland in the brain and stops the production of vasopressin, a hormone that controls the re-absorption of water by the kidneys. With this hormone switched off, our bladders fill up with water and we expel more water than we take in (this is why you spend half the night running to the loo after a few drinks). The body and its organs need water to work properly and, without it, the cells sort of begin to shrivel - yup, that includes your brain cells. Our brain is attached to the skull via filaments and, as it shrinks, it pulls on these filaments and we get a pounding headache. Your mouth also dries out and bacteria can build up, leaving a foul tasting scum on our tongues.
It's not just ethanol that causes our hangover - there are usually other nasty things in alcoholic drinks (unless you are particularly hardcore and drink neat ethanol). To start with, ethanol isn't the only alcohol in beer, wine and spirits. Methanol can be present, particularly in cheap wine and this, unlike ethanol, is a poison to humans. When the liver breaks down methanol, formic acid is formed - a really nasty chemical that you don't want hanging around in your body.
Red wine also contains tannins that can give you a nasty headache if you drink too much. But tannins aren't really too bad, its all the other flavorings and impurities in drinks you need to worry about! Arsenic is even found in some drinks! The worst impurity offenders are cheap red wine, port and brandy - these drinks are responsible for really bad hangovers so it's best to stick to the clear drinks such as vodka and white wine.
You know how people tell you not to mix drinks? Well there is some logic behind this as impurities in different drinks can react and make you feel pretty rough. The impurities and toxins can also make your stomach a little unhappy, explaining why you sometimes throw up the next morning and generally feel pretty sick.
Another cause of a hangover is sleep deprivation. Our brains don't work as well with lots of alcohol flying about in them and we are unable to enter the REM stage of sleep properly and wake up feeling really tired and irritated. Loss of salt and sugar can also play a big part. My favorite cure for a hangover is lots of fried food - I'm not sure how this works but it usually makes me feel vaguely human again!