Economists, those rational devils, are fond of saying it is perfectly rational not to vote. Why do they say this? They'll claim that your vote really is not decisive, that are too much costs in the way of time incurred actually voting or figuring out which candidate/party to support. Plus, they'll say the marginal utility received in one candidate winning over the other is so small, that it doesn't really matter who wins. (You get the feeling economists need to get out of their office more?)
What economists can't adequately measure are two things. First, a sense of civic duty/obligation that most voters have. Second, they cannot measure the intrinsic value in participating in these annual (or biannual or quadrennial) exercises in the public good. True, your individual vote isn't likely to determine a election, but in the aggregate, if millions of us take that attitude (and unfortunately millions do), it can affect outcomes, and therefore policies, and therefore whether or not we go to war and a hundred other outcomes.