Because human behavior boils down to Plato vs Aristotle. If you don't read both, then there is a segment of the population who's actions and opinions are going to be a complete mystery to you.
This is one the most unfortunate things I commonly run into when reading philosophy (or talking with those who study it): imagine if once physics became an established, separate scientific endeavor from philosophy (though it was once a part of it), and yet the majority of philosophers kept on inventing their own versions of physics (without ever doing experimental verification) and pretending the scientific branch didn't exist. I think this is a fairly accurate analogy for the relationship between psychology and philosophy.
To be certain there are many subjects bordering on psychology which are still correctly in the domain of philosophy (e.g. free will or the 'hard problem' of consciousness), but then there are many more mundane issues which are already basically settled in psychology which philosophers still treat as if they were some deep fundamental aspect of reality.
I suppose part of the difficulty is finding quality material in psychology, since it's a vast field with many sub-disciplines. That said, the search can be richly rewarding. There are experiments described in Gazzaniga's The Social Brain, for example, which profoundly changed my understanding of the human brain (and have remained consistent with much more reading over the decade following).
Many of my professors loved the idea of the cargo cult, the horror and glory of it. So, don't be fooled: You may discover a bit of cargo cult within modern studies of Plato and Aristotle.
For one thing, Plato and Aristotle both assert that there's some sort of absolute truth or order to the world, but many other human traditions reject this idea and offer a variety of counterpoints. My favorite is that the questions of material/immaterial, monism/dualism are irrelevant, and that the universe is presided over by tricksters of various sorts with almost nothing to do with the co-occurring human world. There's Coyote, Loki, Anansi, Hermes, many mythical characters from across traditions who would have both Plato and Aristotle know they're merely daft professors, and there's no such thing as atoms nor caves.
These ideas and their idealized past always center on the idea of a natural human order. This arises from, in Plato, the idea that each mind is imprisoned in a series of closed cabinets, and can achieve perfect knowledge of neither truth nor justice; in Aristotle, from a first-moving cause within a universe in which all things are bound by a certain causality. Rome utilized an initiate religion which was used to structure business hours, and you can trivially extrapolate to Darwin and Victorian society, or wherever you like. And, generally speaking, things aren't great for people without the "advanced literacy." Philosophers from around the same era would lampoon all of these notions: Heraclitus insisted there was no cause and effect, Diogenes insisted there could be neither ideals nor wisdom. Even today you can find physicists who are skeptical of causality outside of controlled experiments.
When you look at alternatives to the Great Civilizations of the West and Near East, many of them vanished without written record (I wrote from my laptop, as a technology professional) or created massive citadels and then abandoned them the way one leaves behind sand castles (common throughout the Americas). The apocalypse prophecy of the Hopi tradition, Koyaanisqatsi, states that the world ends when people begin to mine for minerals. Note that after the fall of the Roman Empire, steel was lost to civilization (outside of China? Can't remember) for a long time. But of course, you can think of perhaps ten traditions, without using Google, where the end times have always been upon us.
To place faith in one particular myth, philosophy, or history as always defining of an individual person or their world, or somehow superior to another, is to err. Individual world-theories are often the o-rings of our society, to be puzzled out by inspectors like Feynman long after they fail catastrophically, but in other places their graceful interaction can indeed change the world forever and for the better.
Not sure what all these words will mean to you but I hope they helped you understand where my mind has wandered lately.
This is correct and the fundamental that distinguishes these two thinkers is the Primacy of Consciousness versus the Primacy of Existence, i.e., subjectivity versus objectivity. Kant's theories are a secularized, modernized version of Plato's ideas (i.e., removed the supernatural elements and replaced God with Society) whereas Rand's theories are a revised and more consistent version of Aristotle's ideas (i.e., consistently applied the law of identity to all branches of philosophy and especially to consciousness itself).
So the two poles of Western thought guiding human behavior are Plato/Kant versus Aristotle/Rand.
e.g.
"Throughout modern times, practically every advance in science, in logic, or in philosophy has had to be made in the teeth of the opposition from Aristotle's disciples."
"Aristotle, it should be said, has been one of the great misfortunes of the human race." (Scientific outlook)
In each of us there are four internal voices: sensation, rational thought, emotion and intuition. When you choose to listen to one or more of these voices you get a philosophical school. Plato is mainly intuitive with a touch of emotion. Aristotle is mainly rational with a touch of intuition. They're both western philosophers because eastern philosophy focuses a lot on the senses.
Here are some pure philosophies for each of the four voices, so you can get an idea of what I'm talking about:
* Sensation: Taoism
* Rational thought: Rationalism
* Emotion: Romanticism
* Intuition: Mysticism
However, philosophies are rarely pure, which is why there are so many of them.
[1]: https://en.wikipedia.org/wiki/Big_Five_personality_traits
You're claiming human behavior is much more complex than this. Fine.
How do you know it to be more complex? Through objective sociological methods? Or "just" because of what you intuit?