The same day that I started seriously reading Plato, I found myself involved in a not-so-purely-philosophical dilemma. I don't want to go into detail about that, but I do want to say this: I understand now both why the rationalistic approach of Greek philosophy was so valuable, and also why so many people find it unsatisfactory.
There may be some people for whom instinct is a sure guide to behaviour. But for everyone else, what happens when your instincts freeze up? When they're unclear? What happens if your instincts are based on some defect in your own nature? If your instincts are telling you to take the easier path, are they giving you good advice, or are you just looking for an excuse? Most importantly, and, in my experience, most commonly: what happens if two instincts are at war? What do you do when faced with the paralysis of emotion and intellect which results?
Rationality is great tool in these situations. It makes things clearer. It can guide you to what is right. Then it's just a question of doing it. Rationality is, in the end, calming.
On the other hand, consider this premise from Protagoras: "[E]verything has one opposite and not more than one[.]" Now, it's not clear that this had actually been proven during the preceding dialogue. Nevertheless, it's a dictum that Plato uses as essentially axiomatic: to everything its opposite, and only one opposite. It's a binary approach to the world. A backslash approach. On/off. Good/evil. Pain/pleasure.
Real life doesn't operate in this way. It can't be reduced to this simple a level. The axiom is false; not an absolute falsehood, but a reductio ad absurdem of lived experience.
So the positive virtue of rationality, its ability to clear away clutter and present incisive solutions to muddled problems, becomes a vice when taken too far: the tendency to reduce reality to simple and unreal abstractions. One might conclude that rationality's main characteristic, then, is its tendency to concentrate, to distill all things to their essence — even if the essence does not exist, or if the essence resides in precisely what is inessential.
That ability to reduce by means of the intellect is something tremendously useful. Also tremendously dangerous; people go to war over abstractions. But they also go to war over their instincts. Reason's ability to clarify represents a tremendous leap forward for human cognition; even, perhaps, a movement to something more purely human. That is, if what humans do better than any other known animal is think, then the means of focussing thought enhance one's ability to be characteristically human. But that's a great "if"; what about the ability to act in accord with a moral nature? Or to create art? In both cases the intellect is involved, but in both cases something else, some other faculty, appears involved as well. Reason, in other words, has its limits.
There have been times in history — many of them — where more rationality would have considerably helped the world. But it is possible that too much rationality may be as dangerous as too little.