a couple reasons – the water itself, and you sipping on said water.
for the water, fresh water isnt entirely 100% clean. it is typically going to have at least some bacteria, minerals, etc in it. not enough to hurt you of course, but it is still there. over the course of days and days, the bacterial colonies will eventually grow, expel whatever toxins depending on the type, etc
in terms of you sipping on the water, your mouth if full of bacteria. even if you just brushed your teeth, its there. every time you sip your bottle, some of that bacteria is transferring to your bottle. as previously stated, that bacteria will reproduce and do some other not so fun stuff
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