Why does no-one smile anymore?

On the whole, I enjoy my running. I say on the whole because, like most of us, I occassionally have a day when it just isn't fun. Some days you really feel like putting your feet up in front of the telly and having another glass of wine or there are those days when you're out running but for whatever reason it is just not happening; you feel sluggish, tired and not overly keen on the whole thing.
Running is, however, my chosen pastime and so I do it because I enjoy it. When I am out running I am usually in a happy place in a strange pain filled kind of way.
It would appear however, that many of my fellow runners do not share this joy - for example:
I was running into work this morning and as usual, having to follow the less than scenic route down to Anniesland Cross, along Great Western Road and in through the West End to my office. On the way I passed several other runners of various sizes, shapes and abilities. When I pass another runner I will look across at them, raise my hand and say "hi" or "morning" or some other similar single word greeting. Without fail, not one of them returns my salutation. The responses range from a look of horror that I have had the cheek to speak to them to complete embarassment that someone has spotted them out running. Not one of them even smiles or acknowledges my presence.
Now I'm not looking to engage them in a lengthy conversation or ask them some personal question, it is merely exchanging pleasantries between two like minded individuals. A common acknowledgement of a shared experience.
Do these people not enjoy what they are doing? Are they all enduring some terrible internal pain that renders the whole process so unbearable that they can't even raise a smile - Stop running then! Stay at home and be miserable in front of the telly.
One thing I can't accuse my fellow ultra runners of is a lack of friendlyness (?) There is a camaraderie between runners when you've been out for 6 hours and your goal is merely to get to the end. There is no competetive sneering or dirty looks but there is always an encouraging comment or a warm hand shake and, of course, a smile!