Not the original poster, but I'd imagine something similar to gay marriage; after decades of "it'll collapse society if people can do x" warnings, it becomes clear they were wildly overblown.
I don't think that is what they were implying. Conservatism tends to be about not changing or considering new things. It tends to be more "I was raised that X was wrong, so X is wrong". (It is how I was raised for sure.) So, if a historically conservative person tries weed and it doesn't turn out to be the all encompassing evil they were lead to believe, it may open them up to forming new personally verified opinions on other issues too.
Psychological trait: Openness. It's associated with Liberalism and the political left.
A lot of my right of center friends have seemed to justify drug use just fine these days.
That's the needle moving.
yeah, libertarianism/classic liberalism used to be considered left-wing in the usa, but now is a right-of-center position.
You've got it backwards.
It's a feedback loop.
If you do drugs, you think politically left?
Those who think politically left do drugs?
Yeah, I don't think it comes down to left/right as much as the orthogonal spectrum of conservativism to (small l) libertarianism. People across the left/right spectrum are anti-drug for public safety, decency, or religious reasons, but equally there are those who feel individuals should be able to do what they want to themselves.