I have always found it surprising that someone may honestly think that upvoting and downvoting can be used to express agreement or disagreement. If you strongly agree or disagree, just reply with a post of your own; up/downvoting is there so you could express your opinion on the quality of the post rather than whether you think that what it is saying is true or false.
Paul Graham, who started HN, is one of those people: https://news.ycombinator.com/item?id=117171 . Generally, HN strongly discourages "me too" or "i agree" type comments. We prefer to just upvote comments in order to keep discussion threads interesting and on-topic.
It's pretty much upvote to agree, downvote if it doesn't contribute.
Of course the next highest child is then always the next most agreed upon counter argument since they should reply instead of downvote, which can make HN feel a bit contrary but full of good discourse
