Don't just assume someone isn't friendly, I for one wouldn't always nod if I'm in heavy traffic because my attention is all being put into staying safe. If that means I don't nod to some bloke that can afford to pay attention to me, then so be it. Maybe when I get more experienced I'd have time to spare some attention to fit a nod in even in heavy traffic. Mostly I do nod, but just sometimes I don't and then if I get a nod and notice it then it's normally too late to nod back and I think that the rider probably think I'm an Donkey after not getting a nod in return.
ps. I've given up nodding to moped/scooter riders, there's just something wrong with them, their eyes can only look strait ahead.