I grew up drinking almost exclusively Coke (thanks mom and dad!), and it took me a very long time to realize that my "craving" for Coke was actually just thirst.

These days I try to stick exclusively to drinking water, but I definitely understand how people can be chronically dehydrated like that when their body is so conditioned to drinking garbage.

To this day, I'll still occasionally think "man, I could really go for a can of Coke," but then I drink water and it goes away.

Some of us have been failed by biology and never get thirsty. Makes you seem kinda stupid sometimes when you go "Huh, why do I have a headache and a dry mouth? Oh, right, I haven't had anything to drink since yesterday."

I have ADHD. This has two consequences:

1. My ADHD meds are a diuretic, so on days when I take them I expel more water than I otherwise would. Making matters worse, I feel like when I take them I often just don't really get thirsty. As a result, if I don't pretty actively remind myself to drink water then I can easily find myself only noticing that I'm thirsty when I'm noticeably pretty dehydrated. Like, "wtf why are my eyes so dry? Oh right I don't think I've had any water since like yesterday morning".
2. On days when I don't take my meds (weekends), I might notice I'm thirsty but then just get distracted by something and forget about it for hours. The same applies to a lot of other bodily needs. No, I don't know how I've survived this long.

I don't get thirsty. at all. I need to actively remind myself to drink water or else I get a splitting headache after a while

As people age, their thirst reflex tends to diminish or reduce. There are a few reasons for this:

1. The hypothalamus, the part of the brain that regulates body functions like thirst, becomes less sensitive to changes in the body's water levels. As a result, older adults do not feel thirsty even when their bodies need more water.

2. Kidneys become less effective at concentrating urine as one ages. This means older kidneys lose more water during the process of filtering and eliminating waste from the blood. However, the reduced thirst reflex does not signal the body to drink more water to compensate for this loss.

3. Levels of hormones like vasopressin, which help the kidneys conserve water, tend to decrease with age. This further impairs the kidneys' ability to retain water and increases the risk of dehydration if fluid intake is not sufficient.

4. Physical changes like loss of muscle mass and a slower metabolism reduce the overall fluid needs of the body. But the thirst mechanism is not adjusted accordingly, leading to inadequate fluid intake if one relies solely on thirst.

In summary, age-related physiological changes in the brain and kidneys undermine the body's ability to sense dehydration through thirst. This is why it is important for older adults to drink water regularly, even when not feeling thirsty, to maintain proper hydration.

In a personal note, at 40 years old, in noticing this effect already.

I work a desk job and I used to be too lazy to get up and get a drink, and I tend to also not notice when really focused on something. When I'm outside, I notice the thirst a lot more, but in a climate controlled room sitting at a desk, it's less urgent.

So I keep a bottle next to me all day and refill when I empty it every couple hours. I don't count how much I drink, and I don't think the "8 glasses every day" thing is particularly relevant, but I am plenty hydrated throughout the day (probably something like 6 cups of whatever without thinking about it, maybe more).

Everyone's experiences are subjective. Some people experience thirst as hunger for instance.

Yeah but then I’ll get a class of water, set it down on my desk and proceed to not drink it for another couple of hours.

Surprisingly, you can get dehydrated and not thirsty. I've had this happen. Not often, but it does happen. Usually, my caffeine addiction makes sure that I get enough hydration, though.

So as someone who doesn't get thirsty and actively has to be reminded to drink water, here's my theory.

1. The temperature of the water matters just enough that it changes if I want to drink it, but not so much that I'm willing to make a specific temperature of water. E.g. cold drink on a cold day? No thanks, but I'm not heating up some water for a glass that's right.
2. I eat a lot of juicy foods like fruit and some meats. Even some foods with sauces. I eat a good amount of my hydration
3. Water doesn't taste good and has a distinct lack of texture that I just don't jive with
4. I HATE having to go pee too often. If I could never go to the bathroom that is what I would choose. So I may have unintentionally found a rhythm that keeps me relatively hydrated (I don't get thirsty and my lips aren't usually dry or anything).

That said, I have recently started making fruit juice that is like 80% water and 20% whole fruit and its helped my bowel movements a lot.