We are not, because he could also have had contact with particularly hot sauce, or an allergy, or an influenza infection, or covid, or early stages of worse things. There is a reason why we train medical doctors for years before they start diagnosing people.
Yet, people can and will use heuristics as GP does to make such inferences all the time. I don’t think we can help it. Whether that’s a problem depends on what one does with such an inference.
In the case of a suspected cold, it’s perfectly reasonable to keep one’s distance to try to avoid catching it, whether the supposition is correct or not.
Suspected depression? Harder to say, but at minimum I hope that appraisal engenders understanding and compassion. I think gently suggesting someone seek professional help is reasonable. I also think we have a tendency to overly pathologize perfectly normal mental states, however, and especially if (as the OP states) a person appraises themselves to be OK, then suggestion should not become insistence.
And when someone drops a brick, and it falls to the floor, unless you're a physicist, you shouldn't infer it fell because of gravity. It could be that the video was taken in a room in space, and it feel because the space-ship is accelerating, or due to centrifugal forces from artificial gravity, or the brick has metal components, and there's a magnet under it.
> There is a reason why we train medical doctors for years before they start diagnosing people.
That reason is partly so that they can be recognized as doctors. Making an observational deduction has nothing to do with medicine nor imitating a doctor, per se.
> There is a reason why we train medical doctors for years before they start diagnosing people.
This is not support for your previous sentence. A large portion of doctors' training consists of making sure they will treat someone with a runny nose for a cold before they'll even consider other explanations.
If someone writes that he is sneezing, has a sore throat, runny nose etc., are we allowed to infer that he has a cold?