Voting is a protected right. Forcing a person to vote will no longer allow for voting to be a protected right. Making not voting Illegal infringes on citizens liberty. persons should not be required to make a decision if they do not agree with any options.
without full participation, accuracy of results can be manipulated by various forms of voter disenfranchisement or ballot box stuffing. Requirement implies verification of participation, which should eliminate or reduce those factors.
what is the purpose of voting? It presumes that all those who vote have enough knowledge about politics to vote for whatever brings the optimal greater good. It seems counterintuitive to force someone with no knowledge or interest in politics to vote.
Democracy is rule by the average and the media. The average person is uninformed about most things, and more concerned with their daily lives, and will vote for whoever gives them the most 'free' stuff or makes them feel good.