If I release a single molecule of a gas into a vacuum chamber, I have no idea where it will be at any given moment. No clue what that molecule is going to do. But if I release a volume of gas equal to the volume of the chamber (or half, or double, or whatever) I have a pretty good idea what the condition of that chamber will be at any given time. In fact, I can mathematically model it to such a degree of precision that I could see all kinds of practical outcomes.
People? Similar in many ways. One person -- I don't know what he's going to do. One visitor to my website, one guy who sees my TV ad, whatever. But the stream of people who visit my site? Much more predictable.
The difference between people and gases is that people are unique. That uniqueness means I can't predict exactly how successful my Superbowl spot will be. But the uniqueness is also significantly diminished as lots of people are lumped together. The common elements of people start to stand out more.
And that's where mobs come from. And TV audiences. The common elements are also often the base elements. Dumb, selfish, and lazy.
That's why when I analyze a website, even if not one of the visitors will be dumb, selfish, or lazy, I know that they all will be. (No offense.)