why does everyone you know take a bottle of water with them everywhere they go?
Because the big international bottled water companies have played a clever advertising trick on them. Many people drink far more water than they need now, which is mostly harmless, in itself. Although as a PP pointed out, water overdose can be life threatening, the biggest danger is stomach upsets from bacteria in the water bottles. And of course, the inconvenience of needing to go to the toilet more often, as your body deals with the excess of water. ( And this is a massive inconvenience in schools, where many children are constantly drinking far too much water)
But why was this advertising trick so successful? Psychologists have theorised that it is to do with the instinct for oral comfort, like sucking your thumb, or a baby sucking a dummy.
I don't carry a water bottle, and never have, and never will. I run marathons without carrying a water bottle. I come from the generation BEFORE bottled water was pushed in your face so much. We used to have a drink of water after breakfast, and after tea in the evening. There was supposed to be water available at school lunches, but there wasn't always. There would be drinks available if it was very hot, or if we were doing something extra strenuous, hiking up a mountain, or something. In other words, if it was likely to actually be necessary.