For me receiving and giving an acknowledgment to other riders on the road makes the training more enjoyable. I say hello to joggers, dog walkers and horse riders as well. On the other hand when I get ignored it can be a touch annoying.
On one occasion when I felt particularly grumpy I acknowledged a rider going the other way and was met with an aggressive stare that followed me as we passed. Stupidly I did a U turn , caught him up, overtook him and expecting him to repeat his stare routine gave him one back. Instead he gave me a cheery hello. Felt a bit of a burke and rode on as fast as I could.