Hold the conversation....please

There is nothing more disturbing and uncomfortable than when people I don't know try to have I conversation with me. It usually starts with me being some quiet new and then as usual, I sit quietly having 2nd and 3rd place conversations in my head.
All of a sudden, someone from no where appears sits beside me, which on its own is quiet uncomfortable. This fellow not realizing the uncomfortable corner that he or she has shoved me into, then goes a step further and says "hi". With this, it seems all hell is then let loose, the gitters then begin to start, then gradually I feel the temperature rise until I feel the beads of sweat running down my forehead...and all I could do is give a plastic smile and nod with occasional voicing of "hmmmm".
When the conversation comes to an end and the individual finally takes his or her leave, I breathe a sigh of relief but still I get disturbed that that fellow would have left with the impression that I am a proud, egotistic snob.
...oh well...that's her/his problem now...