[phyller]

Well usually everyone around you means you no harm. The person who looks at you and doesn't smile is legitimately more likely to not care if you get hit by a bus or not, even if they have no ill intention towards you. The crazy guy whose alternate personality is telling him to stab you is definitely not smiling but staring at you and muttering to himself.

Of course context matters. I don't trust a smile from a used car salesman. I trust a smile more from someone I am playing basketball with, or that I pass while hiking in the woods as they have no incentive to deceive me. And if I don't get a smile from someone I pass while hiking in the woods I would be more on my guard.