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Motives

I'm very analytical in all aspects that I have the opportunity to be as such.
I like to try and read people as much as I can to understand their motives. Often I can get a pretty good reading on people and their motives. When I'm very unsure of a persons motives, I get very uncomfortable.

Today, during lecture, someone I did not know sat down in the chair next to me, on the side closest to the entrance to aisle. This broke the cardinal room of seating, never sit next to anyone without a very good reason.
Now this happens normally on the first couple of days of class, and it is normal. It also makes sense if you're trying to sit next to someone you know, have special needs, etc.
Now, she sat on the spot next to me closer to the exit, but not next to anyone. The person never engaged with anyone in conversation and so forth.

Eventually I did find out the persons motives, but not before I was distracted from the entire lecture trying to pin what the person wanted down.

The main question is this, why was I so uncomfortable with someone sitting next to me that I didn't know?
Why does our society have these unwritten rules?