If you're a legal resident, 18 years-of-age+, and not incarcerated or on parole, there's no excuse for not being a registered voter ... who actually votes. Voting is not only the kind thing to do, sometimes not voting actually makes you sort of a dick, here's why:
People fought and died for the right to vote
People wanted the right to vote so badly, they sacrificed their life in the fight to give you the right to do so. And there's zero fighting or dying involved in your vote. You got off easy. So, just do it. Don't be a dick.
In local elections, your vote legitimately matters
National elections cannot save your schools, libraries, parks or put your governor or mayor into office - that's all local. And there have been cases where elections came down to a few votes. So yours definitely counts. You're important.
Your vote can help those unable to vote
Mentally ill, those incarcerated, children, non-citizens, animals, housing rights, the environment, etc., etc. Without your voice, they'll be spoken for by someone else. And that person might suck.
Grassroots campaigns work. Screw just accepting the way things are because you believe you're powerless. Most progressive changes start small, and locally. Join in.
You earn the right to bitch
If you complain about something, but do nothing - you're annoying and a whiner.