Does the same apply to upvotes? If not, why?
Perhaps, but I think the bigger issue is that downvotes can make your comment illegible. If you spend any time on Reddit, you'll know that the vote system pushes down any comments that go against the grain and reinforces the echo chamber effect. Upvotes can do that too, but to a lesser extent.
Personally, I much prefer linear discussions with hidden favorites, like Metafilter.
Use your words.
(This was in response to my comment getting downvotes.)