I normally smile if eye-contact with another photographer. It is nice to see another photographer & see what equipment even whether it is a compact camera or a DSLR. whether amateur or pro. We share the same interest & appreciate the hobby.
Was that "smileless" photographer with wife & kids?
If so, perhaps the hobby crashed with wife that day......quarrelled & black face....& the mind goes "!@#$%^&*)(" like a faulty TV set with "noise" signal only.
It can be a big challenge outing with wife, kids & bag of Camera & lenses (perhaps with tripod).
Still, I think there are some people who are really "songmong" or "social-handicaps" who don't know how to smile & be gracious. please forgive them.