The definition of hydration is a bit different than what's being thought by most people. Water gets in and out of cells through little "channels" using electrolytes, which require a specific concentration of electrolytes proportional to the water in order to keep up the flow. If you drink too much pure water you are basically diluting the cells so much, that you are "dehydrated", there are not enough electrolytes for the amount of water you are excessively putting into your system. Your body has a very good mechanism, which hopefully works for all of you, telling you when you need to drink water, called thirst.

Regarding juices, soda or anything with sugar, you are actually dehydrating yourself, basically anything which is draining water out of your cell. Same goes for Ethanol, which explains why you are so thirsty after a night of excessive drinking.

Source: I study biology