One of the interesting things about voting is that there isn't a good reason for it, especially from the perspective of modeling human behavior that's common in fields like economics. In order to illustrate why this is true, I've put today's Presidential election into a simple game theory framework:
I've modeled both an Obama voter and a Romney voter, but the models are the same. The idea here is that the best payoff occurs when a voter's candidate wins. However, there is also a cost to voting - time, lost wages, travel expenses, psychic cost - represented by c. We've assumed that cost is variable, but less than the payoff of getting one's candidate in office. The result is that the best payoff for each voter is to not vote, but still end up with their candidate winning.
Doesn't voting actually increase the likelihood that your candidate will win? Not really. The closest Presidential election was Kennedy over Nixon in 1960. Kennedy won by just over 100k votes nationally. Even then, that means the odds of an individual vote being the deciding vote is less than 0.00001%. It's usually even less. Given that your vote is incredibly unlikely to decide the outcome, you're always better off not voting because you avoid the costs associated with voting
You might quibble that this model doesn't account for the electoral college, but that makes things worst for most voters. There is absolutely no rational incentive to vote for President in states like California or Texas. Of course, the electoral college does amplify the effect of voting in swing states, but it doesn't really improve the odds - situations like Florida in 2000 notwithstanding.
So, why do so many people vote? There are some possible explanations. Satoshi Kanazawa offers the possibility that we are not in fact forward-looking utility maximizers, but backward-looking adaptive learners, meaning that we're really voting on the last election. Fighting the last war anyone? It also has to do with feeling like a loser or like a winner depending on whether you feel your behavior was rewarded in the past.
Whatever, the reason - and it sure isn't a rational belief that your actions will affect the outcome of the election, because they objectively will not - millions of people still turn out to vote. So, sound off here and let us know why you're jamming a stick in the eye of homo economicus today.