Let's say there's an issue which has 75% for and 25% against. To simplify the numbers we'll say that there are 20 people voting, so 15 for and 5 against. Obviously the bill will pass with a large majority so there's no reason to worry, right? Wrong.
Now let's say prior to the day of voting the 15 people for know each other in groups of 5. Everyone in each of those groups know that 4 other people are going to be voting, and they hear of the same situation in the other groups, so one or more of them may decide that since they "know" at least four other people are supposedly going to be voting it's okay if something comes up and they don't actually vote. Now let's say only one person in each group of five people actually gets their vote in. Assuming that only one person in each group actually votes we now have 3 people voting for.
Obviously social lines aren't so easily delineated so let's broaden the example and show how we could potentially get the same numbers from a broader view. Let's say that each person in the group of 15 knows 4 other people in the same group, but not necessarily the same ones (We'll consider knowledge of those against as bonus relations for simplicity sake). They all express their opinions to each other in favor of the vote so they're all very well ensured that the vote will go in their favor. 10 of the people probably know of at least one of the 5 that will actually vote. But then on election day not everyone who really wanted to vote actually gets to the polls, so on a pessimistic count only 3 people get to the vote while the other 10 'know' that a majority of their friends support it and know at least one person (all too likely the same one person) who will actually be voting.
The people against the issue know they're in the minority and so convince more of their numbers to turn out, so 4 of their 5 do vote.
In the end the vote fails as 4 people have voted against while only 3 have voted for.
And this, dear friends, is why voting is so important. Not just for president, but also for governor, council member, or local state or city issues that affect you at a much more local level. The minority will get vocal and will get a higher percentage of their numbers to turn out so it's important to let your voice be heard.