(1) It's your civic duty. Really? .. In certain instances you might even have an obligation not to vote. If you sincerely believe the system is rigged, as Bernie Sanders and Donald Trump claim, then maybe you should abstain—lest you grant legitimacy to a system that doesn't deserve it.
(2) Not voting insults our veterans who fought for that right .. rights also entails a corresponding right: the right not to do those things .. Veterans fought (and troops now are fighting) to keep Americans free, not to keep them tied down with endless obligations.
(3) If you don't vote, you can't complain. Sure you can ..
Despite the lousy choices and long odds, many of us vote anyway, because .. It's fun to take part in something even when the results are mostly out of our hands. But .. it's perfectly rational to decline—and no moral stain on you if you do.