First Reason: Voting is not an economic use of your time, unless there is a high likelihood that your vote will be a tie-breaker and decide an election. In other words, don't vote if you can do other things with your time that have a higher payoff.
On a personal note, a better use of my time over the decades has been to influence public opinion by publishing hundreds of commentaries in newspapers and professional journals, which have been read by millions of people and no doubt changed the minds of at least a few. I also headed a large grassroots group that struck fear in politicians and got them to make needed changes. By contrast, my one vote has never changed anything.
Second Reason: Your vote isn't going to overcome the overwhelming ignorance of the majority of voters.
For example, about half of voters don't know how many senators there are per state and can't name even one of their senators. As issues get more complex, even fewer voters have the knowledge to cast an informed vote. Consequently, they oftentimes cast a vote that not only hurts themselves but hurts society at large. A case in point: On the issue of trade, the majority of voters today favor protectionism.
Granted, voters are less ignorant about certain litmus-test issues, such as abortion or gun rights, where they tend to know which candidate supports their position, usually due to information and contributions from lobbying groups, such as Planned Parenthood or the National Rifle Association. Voters also are less ignorant when they have a narrow economic interest at stake, such as public school teachers who vote their union's party line in order to keep their government rice bowl.
A citizen who is not part of such a special-interest group or lobbying organization cannot overcome this concentrated power with his or her vote.
Sometimes citizen ignorance is intentional, or what economists call "rational ignorance." This describes citizens who don't pay attention to politics, because they know that being a truly informed voter takes a lot of time—time that has a low payoff and has a better return if invested in some other activity.
Third Reason: Humans are much more irrational than they know or want to admit, especially this human. As a result of evolution (or creation, if creationism is your thing), we're all hardwired to be tribal, to be leery of foreigners and those who don't look like us, to exaggerate dangers, to imagine threats, to follow the herd, to listen to phony experts, and to obey alpha males and females. Politicians play to this evolutionary, or creationist, psychology by pushing our hot buttons.
Wise people won't let politicians push their hot buttons. Instead, they will save their emotions for family, friends, art, and entertainment; and they will reserve the tickles down their legs for bedtime with their spouse. They also will run the other way as soon as a politician makes them pant, swoon, cry, cheer, or otherwise emote, because they realize that their intellect has given way to their emotions. (The German people weren't very wise in 1933.)
Fourth Reason: All advanced democracies have evolved into social-welfare states, which must be their natural state or they wouldn't have evolved this way. In terms of social policies, advanced democracies are a mix of communitarianism and individualism; in terms of economic policies, they are a mix of socialism and free-market capitalism. Since these traits are also hardwired in humans as a result of evolution or creation, the mix for the nation as a whole isn't going to change much, and if it does, it will only change around the margins and tend to revert back to the mean over a number of election cycles. Therefore picking ideological sides and voting accordingly is largely as waste of time. For sure, it's naïve to think that the libertarian fantasy of complete individual freedom and free markets will ever be achieved.
The good news is that as long as enough property rights survive regardless of the party in power, individual and societal prosperity and advancement will continue, as can be seen even in a one-party nation like China. If these rights were ever to face extinction in the USA due to the ignorance of the majority or to a despot taking over the government, the solution wouldn't be voting. The solution would be to take to the streets or emigrate.
Fifth Reason: The choice in most elections is analogous to choosing a stick in your eye or one in your lower anatomy. For me, that's certainly the case with the upcoming election. Whether Trump or Clinton ends up occupying the Oval Office, the outcome won't be good for me and my family—or, in my opinion, for the nation. This is based on Clinton's history and on Trump's comments, especially his comments about placing a 30% tariff on goods imported from China and Mexico, about building a wall along the US/Mexico border (when immigration from Mexico is waning), and about his spending notions and disregard for deficits. Of course it's possible that this was just campaign rhetoric and he won't follow through, which takes us back to my earlier point about politicians pressing hot buttons to get elected.
Closing Note: Please don't resort to the trope that those who don't vote for either of two badly flawed candidates in November will be responsible if the worse of the two is elected and damages the country. The responsible people will be those who voted in the primaries for the two candidates.