This isn’t some moral argument, or anything like that, it’s a very simple psychological fact. Assuming people are fundamentally rational and intelligent actually improves your critical thinking skills.
By dismissing everything that doesn’t make sense as stupid, or arbitrary or random, we also dismiss scrutiny and inquiry. We dullen our capacity to understand things, because we leave no room to understand them. When you have a fundamental assumption that people are smart, then you must find a way in which to justify it. This is when you are able to solve the mysteries of what people say and do, and often there is a good reason for it.
But moreover, sometimes there is also a bad reason for it. Sometimes you will discover that actions which may seem irrational, ignorant or short sighted are rather predicated on contradictory interests to those of your own. If you ever believed a government did something stupid, then I assure you that they rarely do stupid things.
They just do things that benefit them, and does not benefit you. The notion of this singular national interest, wherein the top and bottom of society share the same needs and goals is one of the most common doctrines people are taught in schools.
What is a waste of taxes for you, is likely someone else’s profit. It is not stupidity that leads governments to bail out banks or to go to war, but rather very rational self-interest. It behooves the political aristocracy to do what is profitable to them, not to you.
Similarly, by assuming people are intelligent we are able to discern lies and contradictions. We can find weaknesses in peoples’ narratives and allegations, and discover the truth. Most perceived stupidity is not without motive.
Even beyond this, perhaps people are not always aware of such motives. Adherents to falsehoods, whether in media, or pseudoscience, or schemes in one form or other are generally serving someone else’s interests, even if they do not realise it. So even there we see rational cause.
And even more useful is that by assuming people are intelligent, and by investigating their motives and needs, we also develop the capacity to reason. By seeing what people want, and by holding them in equal regard, we can more often than not persuade people of good alternatives and recourse.
As such we can only find the intelligence which we assume and see in others, because intelligence is not a product of some arbitrary natural force. It is rather to train the mind in the art of inquiry, and in order to do this, we must make the possibility to inquire.
Never buy into the propaganda that you are surrounded by ignorant and huddled masses, this is a notion of tyranny. It is a notion to render people docile and compliant.
By dismissing the reason of others, we cannot scrutinise. And without scrutiny, there is no democracy. What remains rather is anarchy, in which all things are mystified through the falsehoods of this supposed chaos. Wherein everything happens at random, and as such, cannot be governed by the authority of the people.