The "saying hello"-process turned into some sort of routine for me by now, if it is someone who a family member knows, I will greet that person, if they notice me. But anything beyond the "hello" or "hi" seem very dependent on my day and state. If I am in a crowded place, I'll surely have to make an effort not to forget to respond to a greeting or anything alike, same goes for any other situation with lots of input.
Just last week I went by train (with my brother) to get to a place where I could take photos of certain stuff that I needed. On our way back home when I was already exhausted, we met a "collegue" of my brother (if you can call him that way), it was the first time we met (I think) and I was already so done that I forgot to introduce myself...
I thought about the "Hi" at least and shook his hand, but that was embarrassing since he told me his name. Luckily it was so loud in that train that I (after realizing it) was able to pretend I didn't hear what he said and since he got into a conversation with my brother afterwards, I just didn't bother to repeat that "getting familiar"-process. Well, unless the train only seemed loud to me but not others because I was exhausted... I better don't want to know, would be just more embarrassing in retrospect, if it actually wasn't that loud. 
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Diagnosed with Aspergers.
BSP-errors are awesome.