Any sane person wouldn't presume to thing for themselves on the best course of action in treating their cancer, for instance. The same thing goes for nutrition and health.
Many people think otherwise: they second guess medical experts using their own engineering or scientific knowledge. Sometimes this "seems to work" but it's not clear that it does so in anything but an anecdotal way. Carefully controlled trials and meta-studies are much more effective.
Talking of context though, I think any individual is capable of thinking for themselves when it comes to how much water a day to drink.