You want an honest answer to this? I have probably spoken on this somewhere upthread, but after 400 pages over a year and a half, we shouldn't expect anyone to remember..
The basic answer to this is that humans beings aren't nearly so rational as they like to think.
It would not be inaccurate to say that humans really have two different cognitive systems. One is logical, and it is pretty slow. The other is fast, but associated with emotion and memory, rather than much rational analysis. This second system is there for making snap decisions, from back when we needed to feel fear when we saw a jaguar in the brush, or anger when we had to fight, and so on.
The problem is that this emotional system is also largely sub-conscious. So, when you get some stimulus or information, the emotional system responds first, and then your logical systems chime in and, without realizing it, humans generally pick among the rational responses the one that most closely matches their emotional responses.
And, unfortunately, we don't usually recognize this is happening. Which means that someone who knows what they are doing can manipulate us.