Does your vote change anything? If you didn't vote, would it make a difference to the outcome?... You are one in about 40 000 000 voters!
For me the idea that my knowledge and experience are contributing to a better overall decision is the reason to vote, yet I find it hard to apply knowledge and experience to choosing a politician. After all it is quite difficult to judge how someone may act in the future, in a job they have never done before, reacting to unforeseen circumstances, and much easier to decide your position on issues such as the smoking ban or fox hunting.
... and if I don't have an opinion, why should I vote?