simpsonsshitposting

It's not training. It's quite intuitive. Voting for/against something is common in many scenarios, not just from Reddit. And most people will associate up with good and down with bad.

If you like something, you want to reward it, and if you dislike something, you want to do the opposite.

With the vote buttons right there, it was inevitable this is how it would end up.

Even more logically, people know that higher-voted items get more visibility and lower-voted items get less, so if you like something you're more likely to want others to see it. Therefore, upvote it to send it higher up the tree. And send it down to hide it.