I've offered three bright-line tests for when you can feel entitled to an explanation of what is wrong with your comment.

Notice how your use of the word comment, as though all comments and explanations are equal, strips out the quantitative aspect. I don't think that you can expect people to put more effort into explaining a down vote than the writer of the original comment put into writing it. If you spend five minutes writing a comment that contributes a tangle of confusions to the discussion you are not entitled to have a down-voter spend half an hour on a comment that untangles it all for you.

One the other hand, if you spend some extra time on your post, distinguishing subtle nuances of words, and tagging them, eg free1=gratis, free2=libre, and then make your your point with the tagged words, eg the GNU GPL focuses on free2 and free1 is collateral damage, then you have born much of the burden of untangling the ordinary, boring confusions. It is much less labour for a respondent to explain why he disagrees with you, and he should say.

If I thought my own comment was downvote-worthy, I probably wouldn't have posted it

Right, but that doesn't necessarily mean you should upvote it. Comments like the one above, while they add to the discussion, are not on par with comments that also make good, clear arguments, cite sources, and link to relevant resources.

ETA: Yes, I do realize the tension between this and the notion that I wouldn't post something if I didn't think it was upvote-worthy. If everybody agrees with me on this stuff I'm going to have to go back and un-upvote a lot of my own comments.