It's 4 am, I'm awake when I want to be asleep, and I'm bored.
I almost always say hello to riders I pass or am passed by. If they start a conversation, however, I'll probably go back into my shell, take the next turn to somewhere I've never heard of, or simply stop and wait for them to go away. I just don't have the breath, brainpower or desire to talk to people when I'm on my bike. I don't do it even if I'm on a (very rare) group ride.
I do do the 'pass on the flat, die on the hill' thing because I'm overweight and underfit but reasonably strong, so I can go fairly quick on the flat but as soon as the road introduces verticality, I'm fucked. Conversely, I can pass you really fast on the downhill bits. Sadly, there just aren't enough of those where I live, and those there are have an annoying habit of being associated with uphills. I also have some reasonably nice bikes and a few items of expensive kit (no Castelli, I think), so I probably fit the brief. On those rare occasions I overtake anyone in my bit of very rural Suffolk, I do try to pass with a decent amount of clearance however - and, if there's enough breath to spare, a 'hello'.