Dehydration question

I'm not and I hope I never will be. But something I've been wondering that maybe one of you can answer for me because I keep forgetting to ask my doctor or his staff. Why are we at such a higher risk of dehydration than your average person? I know that everyone, in an ideal world, should get 64oz of water a day and I'm not arguing that I don't need it....but why is it so much more important for us? I'm doing what I can, newly post op, to get all my liquids in and I think I'm doing pretty well...so really it's just a curiosity question.