catalana mentioned that one of the topics they're covering in her classes this semester is why someone should bother to vote in a population with a large number of eligible voters, given the vanishingly small chance that one vote will make a difference. Since this came up at about the moment that she was getting out at the train station, I didn't get a chance to hear the reasonable explanation of why a person should vote.
My unreasonable explanation for why I vote is that I view it as a form of sympathetic magic. If I bother to go out and vote for a candidate or issue that I care about, other voters who agree with me may somehow be stimulated to do the same thing.
Of course, living in Cook County, I have a problem finding voters who agree with me, but that's a different problem altogether. :)