Different respondent (I'm not him). I can't answer to his personal experience, and it's never happened to me, but I've seen this happen to people. When I lived in Williamsburg, probably 3/4 of the people I saw on a daily basis were regular recreational drug users. It's a nihilistic, icky culture and the negative energy is intense. People are flaky and half-there. This isn't the fault of the drugs, I don't think. They'd be just as icky and nihilistic if running on booze, cocaine, and casual sex alone. LSD and pot and shrooms merely fail to improve those people. The danger is that, in the process of getting access to a drug like LSD, you're likely to associate with that sort of people more.

Also, while I don't consider psychedelics "evil" at all, they do make a person more suggestible. If you're surrounded by a rotten culture and bombed-out people, you're going to suck in a lot of negative energy.

The recreational drug use lifestyle is, for most people, pretty awful. Again, I think that the chemicals themselves (if we're talking about psilocybin and LSD) are probably a minor factor. But to get in access, you have to deal with despicable people (such as dealers) and make a bunch of shitty friends who are in access, and there's definitely a lot of the crappy, cliquey behavior associated with American high schools. Oh, and since you're dealing with an scumbag black market, you're not always getting the chemicals that you think you are. And even if you're only interested in exploratory, "spiritual" drug use, you're still surrounded by crass, hedonistic, bombed-out people who also use cocaine (which is an asshole/empty-hedonist drug if there ever was one) and opiates (which can wreck your health and turn you into a lethargic zombie). Most of these people also have undiagnosed mental illnesses (not minor yuppie shit, but severe, unmedicated bipolar and schizophrenia that they are actively making worse) that they are too lazy to take care of [0].

[0] Please don't think I'm a Republican for calling these people "lazy". I'm talking about upper-middle- and upper-class hipsters living on parental funds who've had everything handed to them.

This is, I'd argue, a case where the illegality of these drugs makes them a lot worse in their totality (and that's one reason why I'm really glad to see these compounds being studied for potential beneficial effects, even if I'll probably never again use them). Because LSD is illegal and stigmatized, you have to deal with the dregs of humanity to get it. Now, while LSD and psilocybin may not be long-term harmful under ideal conditions (the jury is still out about that, but evidence suggest that they can do a great deal of good) they are pretty awful when used in the wrong setting... and they are almost never available in the right setting. People who run marathons and write novels and program open-source libraries rarely use LSD, not because they're "above it" (I also know some absolutely wonderful people who've used LSD) but because they're just generally not in access.

Furthermore, these drugs seem to scale poorly. The more often you use them, the less benefit they seem to deliver. I think that it's probably healthy for a normal person to have a psychedelic experience on occasion. It can set a person on a different course, and we all need that on occasion. Dropping acid every weekend is probably not a good idea. When drugs become one's life, or one's lens through which everything good or bad is viewed (note: someone who relates all intense experiences to drugs probably should fucking stop using them)... it gets very unhealthy. At this point, the person has frank psychological problems, has probably lost jobs and friends and relationships, and is exactly the sort of person who will be drawn into the use of drugs that are physically harmful (like coke and heroin).

I hope this helps to explain the pattern. Of course, there's a lot tied into it, and a lot of it's cultural. But some people undergo a subtle shift from self-improvement and learning to "experience chasing", and the chasing turns into escapism, and LSD and psilocybin are rather poor drugs for escape (they intensify life, rather than drowning it out) while alcohol, heroin, and cocaine are good for that. I know many who were irresponsible "psychonauts" in their 20s and turned into boozing alcoholics in their 40s.

All that said, this is just one slice of experience and observation (and a negative one). There are plenty of people I know who use these drugs on occasion and haven't gone wrong or boiled their brains. It's not for me, but I think it's necessarily bad. Legalizing and de-stigmatizing psychedelics would do a great deal of good, in my opinion, for everyone by removing them from toxic cultures.