There can be underlying anxiety issues that make going out like that quite a stressful event, especially at first, and even traumatic if something bad does end

There can be underlying anxiety issues that make going out like that quite a stressful event, especially at first, and even traumatic if something bad does end up happening. Or sensory processing issues associated with autism spectrum disorders that make bright sunlight feel unpleasant. Not to downplay the general idea: certainly for most people who feel a little blue, or even crappy enough for long enough to qualify for a diagnosis of major depression, getting out more is likely to be helpful on its own (and that's why therapists generally recommend it). But to the GP's point, the lack of distinction between people with a more ordinary (but quite painful!) set of problems and people with a bunch of comorbidities that they're likely not aware of is really frustrating, especially if they're using the internet to look for ideas and find a bunch of people generalizing the easy fix that cured their depression to the entire population.