Consider, if you will, two different events in twentieth-century history.
In the aftermath of the First World War in Germany, where fear and chaos reigned, many different political groups struggled to rise above the rubble. Eventually, the Nazi Party came to power and held sway over the country. The Nazis started out as another ragtag band of extremists, but they eventually became the oligarchy of Germany.
The methods they used to gain power were not entirely scrupulous. Their primary point of appeal was that they had a scapegoat— Jews, they said, were responsible for all of Germany's misfortunes over the last few decades. By giving people someone to blame and thus a way to channel their grief and anger, the Nazis became very popular. Amidst all the flag-waving, few thought to question the dubious idea that an ethnic group could be intrinsically evil. And so, millions of people died before the German public realized the magnitude of the evil its soldiers were committing.
The second event was far less dire, but still damaging. In the early fifties, the United States was struggling for dominance with the USSR. The threat of nuclear war hung over the American people. Senator Joe McCarthy took advantage of this latent fear and attained celebrity by claiming that dozens of government employees were involved in a Soviet conspiracy. There was indeed a conspiracy, and some of the accused were indeed guilty of treason, but most were entirely innocent. All kinds of people lost their jobs when they were labeled as Communists and blacklisted. Like the Nazis, McCarthy ruined the lives of random people for personal gain by making baseless accusations.
But what if things had been different? What if, in either instance, the general populace had taken a long, hard look at the idea in question, and judged it? I imagine that neither incidence could've happened, and disaster would've thus been avoided. Neither idea was based upon any sort of fact. Both were simply hammered home and treated as if they were unquestionably true. Unsupported as they are, they quickly crumble under any kind of critical analysis, and anybody who attempts to make a logical argument for them will find themself defending an indefensible position.
This is why reason is necessary.
"Reason" is a rather broad term, encompassing logic, common sense, intelligence, wisdom— basically, every form and quality of rational thought. I embrace all of them, and I think everybody else should, too. The two examples that I gave above, of problems arising from a lack of rational thought on the part of the masses, are hardly unique. Countless times, whether or not a population comes to reasonable conclusions about itself and the rest of the world has directed the course of history.
The ancient Athenians, though very progressively democratic for the time, were on the whole quite sexist; women got the short shrift in both law and culture. What if the Athenians had questioned and subsequently gotten rid of this pointless bigotry? Women's suffrage (and perhaps by extension the whole civil rights movement) might've come to the public mind as soon as the Renaissance, when Europe recognized and celebrated the achievements of the ancients for the first time in millennia.
In the USA, when rock-n'-roll music first became wildly popular, some people were so put off by it that they claimed it caused children to commit suicide or worship unpopular deities. The rest of the country either investigated these ideas and found them to be completely nonsensical or dismissed them out of hand. But what if, instead, the public had assumed they were true and acted accordingly? A genre that we now recognize to be as artistic and culturally valuable as any other would be more or less gone.
With reason, we can identify and stop disasters, both major and minor, before they destroy what we value. Should we fail to criticize and judge our own instincts and the assertions of others, we can allow great harm to be done to the entire human race.
So far, I've been relatively reserved in the points I've made; I imagine that most will agree with them, if not how I apply them. But now I'm going to take this reasoning a step further, beyond what you may be comfortable with. I propose that reason should be applied not just occasionally or often, but always.
The ideas that we take for granted, that we accept unquestioningly, tell more about us than what we ponder about and bicker over, and in the end, they matter more. Every time we make a decision or an argument, we start with a few simple, supposedly reasonable, assumptions; and a good thing too, or we'd spend our lives perpetually reinventing the wheel. The problem is that our assumptions aren't always true or rational.
Consider how, for example, if you happened to visit Mesopotamia circa 3000 BCE and discuss geography with one of the natives, you'd find that they'd instinctually and wholeheartedly (and given what they'd know, logically) believe that the earth is flat. And if you changed the topic to civil rights, they'd probably think you were out of your mind. The truth is, we're generally born as woefully ignorant bigots. If rationally backwards or just plain false ideas like ageism and astrology aren't discouraged by society or dismissed by ourselves, they simply gain a greater foothold in our minds as we mature.
My point is that too often we swallow ideas hook, line, and sinker despite their dubious truthfulness for irrational reasons— in many cases, simply because we were brought up in a world in which nobody challenged them, so don't dream of questioning them. But this is an unhealthy and often unethical practice, and must be stopped.
Consequently, in order to make the right decisions, we must judge and criticize, we must ponder and contemplate— we must think! Should you ever dismiss or accept something without pausing to consider it, especially if someone else urges you not to, you do so at your own peril! You may be indirectly allowing a heinous crime to be committed, or hindering a just cause. Considering that the majority of life doesn't take place in the heat of battle, it's always better to be indecisive and hesitate than jump to conclusions that are off the cliff of reason.
Let me be optimistic and assume that you agree with all of the above. Excellent! By taking the path of reason, you do a great service for mankind. But as with everything worth doing, it's easier said than done. In order to make the greatest use of your ability to reason, you'll need to have proper technique.
First and foremost, you probably have more power over yourself than anything else, so use reason to direct your actions and attitudes in general. Lead a self-examined life; become a philosopher. As I like to say,
Dine with Hume's Fork
And shave with Occam's Razor,
But never gamble
On Pascal's Wager.
Make up your mind about religion: pick one, or one of the myriad non-religious views, or simply decide to be indecisive. But be sure you can back up your position with cold, hard logic. If you live by sheer faith, you won't be able to make a non-hypocritical argument against the existence of the Invisible Pink Unicorn.
Secondly, no matter how uncomfortable a proposition or idea may be, give it a chance and spare it a thought. If you feel so averse to even toying with it, perhaps that's because you realize on some level that's it's probably true. However, this doesn't mean that you need to seriously ponder every little possibility, no matter how remote— once you've reasoned Nessie out of existence, you can dismiss Bigfoot out of hand by simply applying the same reasoning. When dealing with anything paranormal, keep in mind who has the burden of proof.
Finally, whenever you argue, verbally or otherwise, do your best to promote the cause of reason. Obviously, you should only argue for rational positions, but also make sure to keep your arguments themselves reasonable. You can be clever, but don't try to trick, harass, or manipulate your opponent, even if they do so to you or it's "for the greater good"— two wrongs don't make a right, and few goods are greater than reason.