Reasons to vote, limiting the discussion to presidential elections:
1. Exercise your rights.
2. Feel like you've voiced your opinion.
Reasons not to vote:
1. My state is never in play. My state's electoral votes always go to the same party. My vote will never change that. Voting in my state is a waste of time.
2. I dislike both candidates. The last two presidential elections, I could not in good conscience bring myself to vote for either of the candidates.
I once knew a retired Army guy who said, "As soon as they get a candidate worth voting for, I'll go vote." He hadn't voted in decades.