why is cold water more refreshing than warm water?

Because people drink cold water when they're overheated to begin with.

Interesting question without a simple answer. When I'm thirsty but not
overheated, cold water is definitely more refreshing than lukewarm
water. It's not a difference in mineral content either, because that
same glass of cold water that sits out and warms up is not refreshing
either. Must be psychological.

You may know that our body regulates its temperature by many ways.
When you are overheated, the most important mechanism used to put this
heat away is to sweat.
Every gram of water that you weat carries 540 cal of heat when it
evaporates. If we assume that 20 droplets of sweat is equal to 1
mililiter (that weights 1 gram), it is correct to say that every
droplet of sweat will cool you 27 cal down.
Now let's imagine that you drink a 200 mililiter glass of cold water
(let's assume it will be at 10 ºC). When this water hit your
overheated body, it will try to achieve the thermal equilibrium
absorbing heat. Your body will be at 36ºC (assuming you didn't got a
fever) and the cold water temperature will have to rise 26ºC (36ºC -
10ºC). Knowing that every gram of water absorbs 1 cal of heat to
increase 1ºC we can calculate that the 200 mililiter (200 grams) glass
of 10ºC water will absorb 5200 cal of heat. This heat is equivalent to
192 droplets of sweat (or almost 10 mililiters)!!!
Resuming: a 200 ml glass of water @ 10ºC = 192 droplets of sweat
Isn't it refreshing?

The test of rogeriotakejame's hypothesis would be if cold water was
more refreshing than warm water when you are hot, but not when you are
cool.

Most people find any cold fluid is more refreshing than the same fluid hot.
I personally find very hot tea (with NO sugar) to be the most
refreshing on a hot day.
Barrie

Getting all hot and bothered, or someone heating you up, or a heated
debate etc. Vrs cooling off, being cool, playing it cool. Also fried
foods and mainstream diets are usually more acid forming than alcaline
foods which should be more prominant than what they are. Cool or cold
foods and drink, are not considerd healthy by chinese or alternative
therapies. I hate warm beer though, having a cold cider right now
actually. My girlfriend has to sleep with a wet towel on her head, and
never wears warm clothes. I, by comparison need more blankets and
clothes, and hate cold water as a drink, prefering room temperature.
Also I avoid icecream which she loves. I though being male, have a yin
personality, she though female is yang