People are not:
* linear time-invariant
* rational
* abstractable
By the first, I mean two things. People do not react proportionally to a change in input--raising your voice slightly or disagreeing somewhat harder may suddenly get you punched in the nose instead of just yelled back at somewhat more, or being even nicer to somebody may not return any change whatsoever. Moreover, the same input applied at a later time may completely change the reaction of a person--a harmless text message repeated at a later date could set a person off.
By the second, I mean that people simply do not follow a logical chain in most situation. Given a set of rules and inputs, you'd think that a person (and even their mental state) would behave predictably; this is obviously false if you've ever done business or sold to consumers or dated.
By the third (possibly the largest), I mean that people can't be reasoned about at a high level. You simply can't ignore or approximate all of the little minutiae that make up that person's background, and if you try you'll find that everything falls to pieces. "Every girl likes compliments" fails, "every customer wants the cheapest goods" fails, every abstraction falls over in contact with the enemy.
EDIT:
People seem to be reacting negatively to the opening statements of the article. When I read it, I interpreted it as the author trying to express the idea that even massively difficult technical problems are still amendable to standard problem-solving techniques, in implicit contrast to "people problems" which the author finds elusive.
A correct reading, I suggest, would be that the opening is more an attempt at example than braggadocio.