sorry please thank you

I'm sitting on the bus, writing, a sea of empty seats around me, when a man, probably around 40, motions to the seat with my bag on it and asks to sit down. I inwardly curse, because I'm too paranoid to write if there's someone next to me, but I move my bag and smile.
"Thank you," he says. He takes out a tin of mints and suddenly I know what is coming, because I have taken too many buses not to know how this goes.
"Would you like one?"
"Oh, no thank you."
"Do you go to UCLA?"
"No." I literally have to bite down the polite words that answer his real questions: Where do you go? How old are you?
"But you live here?"
"Yes." I stare out the window, then at the empty seats all around us. His hand is casually perched against the side of my leg.
And four or five stops later I get off, incredibly irritated and not entirely sure why.
On one level it's with him, for interrupting me. On another I feel ridiculous for being mad at someone for being friendly. But I'm also irritated at my politeness, an instinct I have that prevents me from being an asshole to strangers.
I'm not sure why, but I am incapable of being an asshole to people who actually deserve it.
Around my family, I am what they sometimes refer to as "snarky" and sometimes as "fresh," but put me around someone I don't know and I will apologize profusely if punched in the face. Am I just trying to be liked? Do I think all strangers are psychotic borderline-personality rapists who will kill me in a dark alley if I don't smile and say Thank you! with the exclamation point?
Neither option is particularly comforting.
But I guess it's a little comforting to know that my sister is the same way. Once, biking to the store, a truck knocked her over and when she got up, bleeding in five places, she said "Thank you."

We talk about it sometimes, wishing to be rude. Wishing we could say what we really feel while simultaneously knowing our nervous, careful paranoia will prevent any such thing.
"It's just the way we are," she says, and hugs me.