Human personality distribution is just another thing that is evolved. There's a natural distribution, descended from early mammal personality - at different times we reward different personalities, and kill off others. People evaluate what risks are worth it in areas like dealing with tribal conflict, with wild animals, nature, etc. They also had a level of neuroticism that was appropriate to make them do the right amount of introspection.
Some traits, like being super brave, can be good or bad. You might kill the mammoth, become chief, and have lots of kids - but a lot of the time you get killed. If the entire tribe is like that, that's bad news for the tribe, too, because they'll spend the whole time fighting internally. But we need some guys like that every once in a while, to take the big risks when necessary. So over time, a reasonable personality distribution evolved, with some conservatives, some risk takers.
Then alcohol came on the scene. Drunk people made bad mistakes, picked inappropriate fights, tried amazing stunts and fell out of the tree, ate poison mushrooms, passed out in the jungle and got eaten by a jaguar, and the risker part of the distribution of personality was killed. This went on and on. Tribes who kept taking alcohol kept doing this - killing off a section of the population every generation. Depending on the availability, alcohol could have varied effects - from minor to completely debilitating if plentiful. We know that even today, genetic susceptibility plays a big part in alcohol problems.
This has been going on for the last few thousand years (hundreds of generations). People with a really bad susceptibility to alcohol have been being taken out of the population. You can see examples of earlier parts of the process when stone-age tribes first encounter alcohol - it has a devastating effect. Even now it's the leading cause of unnatural death.
(It also has a huge positive effect - making people have more children)
In regular life, though, what happens when all the risk-taking people are dead? There was a reason that that trait evolved before, and that niche was open - so there is selection for people who can fill it, without getting themselves killed by alcohol. So, evolution is trying to re-establish a reasonable personality mix, but under a new environment.
It's similar to when a new antibiotic comes on the scene - it may kill 99% of the population, but the population comes back, minus the trait that made it vulnerable. Humans still need risk-takers, but to survive as a risk-taker you have new dangers.
Various traits have evolved as a response to this - hugely increased alcohol resistance, and the ability to break down alcohol quickly is one. I imagine that it's also made people quite a bit more cautious about doing something it considers "slightly dangerous, but worth the risk" - an ancient mind thinking that could be confident of it's sense input. But a mind in a world with alcohol evaluating the same supposition should be a lot more careful, because the mind shouldn't trust it's own perceptions as much.
Once you get used to it, you can't go back. All of our ancestors made it through this - they were the "don't pick a fight with a jaguar" type of tribesperson, the stable one who didn't drink their life away. Or, they were the one who loved to get drunk and become a father. A couple thousand years later, evolution has again selected for specific types of personality, with awareness that they will be acting in a specific environment. You can't just then remove alcohol again and expect things to work again. Risk taking under alcohol is much more dangerous, and we've been selected for resistance to it.
A couple of other examples...
We're selected for an environment with alcohol. (well, some people are - there are also groups which have avoided it).
Ever wonder why the human body can't produce the "essential" amino acids? They're just chemicals, and we can produce lots of them. Other animals can produce them, and we produce all of the "non-essential" ones..
What happened was that we were eating stuff which had them for so long, random mutations broke the machinery for making them spread among humans, and nobody noticed until they took long ocean trips and got scurvy.
Anti-circumcision activists act like they can just stop having it done, and things will definitely be ok. But, for some groups at least, there has been 100+ generations of people doing it continuously. That means that the related genes have been randomly selected and mutating for 2000 years. Would you like to risk suddenly expressing random old genes?