Why do the scales fluctuate so much daily?

I weighed weekly when I was on Weight Watchers many years ago. Some weeks, when I had been very good with both diet and excercise, i wouldn't lose anything, or actually GAIN. It was completely demoralizing. Now that I've been weighing daily for quite a while I have a much better understanding of my body's fluctuations. It took me a while to understand my personal triggers for fluctuations (going up to altitude, a weekend of heavy hiking or backpacking, too much salt, not drinking enough water, too many indulgences, etc.). My rule is, when I see a change on the scale, not to "believe" it until the scale has said the same thing for at least two days in a row. And I am meticulous about weighing under exactly the same conditions every morning (first thing after getting up, after peeing, naked).

It's fun and keeps me focused.