I don't want to talk to you.
This is not a personal affront, because I don't know you, and I'm trying to keep it that way. If I'm sitting next to you on a plane, waiting next to you in the doctor's office, standing next to you in the insanely long line at the post office*, and especially if I'm sitting in the massage chair at the nail place having my toes done, I don't want to talk to you. I'm tired, I'm hungry, I'm probably annoyed at waiting, or if I'm having a pedicure I'm blissful, and I just don't want to chat. If you ask me if I made my scarf, and I politely reply no, that I have no interest in crochet, that is code for "I don't want to have a 30 minue conversation about all the stitches that make up my scarf (since, again, I didn't make it), nor do I care about the knitted blanket your granddaughter got for her last birthday." Like I said, it's nothing personal, but I'm never going to see you again, and I have my own thoughts to deal with.
I don't know why people are compelled to make inane small talk. Uncomfort with silence? The hope that of 100 people, maybe they'll meet 1 really interesting one? I'm willing to take the chance I'm going to miss meeting a really interesting person, because talking makes me tired. I want to read my book, work my crossword, stare into space and daydream. But I don't want to talk to you.
*The post office, by the way, exemplifies every reason monopolies are illegal. Illegal unless the federal government wants to have one, then of course it's perfectly appropriate. I hate the post office.