Parents are told to express disapproval of the actions of their children, rather than of the children themselves. That is because there is a difference between criticizing someone's actions and criticizing them as a person. Otherwise we wouldn't need to read nodes, just look at who wrote them.

In case it appeared any differently, please note I'm not arguing that every or even just every one in ten votes should be explained. (That would silly due to the traffic alone; all we'd be talking about anymore would be votes.) I'm simply stating that most voting is not personal, and that remembering this would help not to take downvotes personally.

I don't feel justified in downvoting a node simply because "it would be better if you did X" in it.

There's difference between "it would be better if you X" and "it is wrong to Y". I won't downvote on the former either, but may on the latter, and will certainly feel compelled to reply and correct it in addition to a downvote. If it has been corrected by someone else, I might just downvote or might not do anything. It really depends on circumstances - my guideline is mostly "will it lead to problems for someone else to take this advice?".