One thing that's easy to miss in the heat of the election, during the onslaught of "vote for me because the other guy is worse" ads that seek to pry away that last few percentage points, is that you don't get to be a major office candidate unless people honestly think you're good for the job. Sure, a lot of people are only voting for Candidate A because they're frightened of what Candidate B would do if elected, and they know their preferred Candidate L would never stand a chance of beating Candidate B. But both A and B got to be where they are because a lot of people were voting FOR them, not AGAINST the rest of the field.
It's almost never a majority, obviously. Or even a very large minority...a lot of it is party inertia, and convincing yourself after the primaries that you're okay with your party's pick even if you were a fervent proponent of one of the losing primary candidates. But you can't even get a serious shot at the nomination unless a lot of people really do like you for the job.
And there's where my above-the-cut point comes in. I expect this is one of those correlations where causation works in both directions.
If you don't think much of your fellow humans, odds are that you'll think their reasons for liking a candidate are stupid. And you're probably not going to like that candidate if he's the Stupid People's Choice.
If you consistently find the party nominees to be unsatisfactory (i.e. you consider yourself a Democrat or Republican, but your choice never even makes it above the margin of error in polls, and the nominee always strikes you as betraying the values that drew you to the party), then you're going to have your opinion of fellow humans lowered every time you go to the polls.
The reverse is probably also true. If you're generally satisfied with at least one of the major party candidates, your tastes are probably in line with those of most voters, and you neither think they're idiots out of the gate nor will come to realize they're idiots or even downright evil.
Because, unfortunately, democracy means that idiots and villains also get a say in government, and dissatisfaction with the results will tend to reinforce the idea that idiots and villains make up the majority of humanity. But seeing humanity as mostly okay probably correlates with being generally okay with one of the major party candidates winning.