Here is Gordon Tullock on why he doesn't vote. He makes good sense.
Tullock's point is that the probability of his vote chaging the outcome of any election is so so so small, that the expected value he would obtain from his preferred candidate winning pales in comparision to the cost of going to the polls, waiting in line, becoming informed, etc. In other words, P•B<C, where P is the probability of any vote changing the outcome of an election, B is the additional benefit one receives from their candidate winning, and C is the cost incurred in voting. Why vote?
Economists and political scientists consider other factors as well, including one's obligation or duty to vote. In other words, P•B + D > C.
We now have to include one other benefit of voting: our spouse might run us over with a Jeep if we don't vote. In other words, P•B + D + Our life > C.