Why do we dehydrate so quickly?

Humans get a lot of moisture from the food we eat. With significantly smaller portions we are getting less, and need to rely more on drinking for liquid intake.

This is mostly in your head.  You've just become accustomed to the habit of drinking and the feeling of cool/flavorful drinks in your mouth.  In absolute terms, if you're drinking that much an urinating that often you're actually over hydrated and you're certainly not dropping into dehydration in half an hour.

I guess I should have phrased it as "thirst;" the only real dehydration was when I had a kidney infection last week, which also seemed to come on really fast. I used to be able to ride 6-8 miles before needing a drink, but now I can't get nearly that far before I'm parched, even if I'm just as well-hydrated beforehand.

It ismostlynthe low carbs and low glycogen stored in our muscles. For eb0very gram of glycogen we need 8 gr of water. That's why when we start low carb diet, we can lose first 5-8 lbs in a few days. 

Most of us during weight loss phase post op WLS... We do mostly low carbs, low sodium... 

Even normal non WLS deal with a lot of muscle cramps, low BP.. Etc..