Dehydration is a big part of it, but so is the general breakdown of alcohol in the body. The alcohol breaks down into some harsh compounds like acetaldehyde and formaldehyde, then it breaks down into acetic acid.
Acetic acid is fine, it's the same stuff in vinegar.. the problem is the middle part. The common reaction to those two is sweating, skin flushing, vomiting, and nausea.
Combine it with water loss from drinking other things than water, throwing up, or peeing more, and then add in the two compounds and it's a painful morning.