I hope you live in a country where it isn't mandatory. There are many countries where you have no choice. They will even punish you for not voting. This includes countries like Brazil and Australia. It's usually just a fine, but it's a big hassle. Some people justify compulsory voting by saying that voting is one of a citizen's duties. If you was the rights of a citizen you must fulfill the duties. There are many arguments against it too though. You can check out the Wikipedia article: https://en.wikipedia.org/wiki/Compulsory_voting