Instead of trying to speak for anybody else, I'll just say when I downvote, and I'm prepared to be blasted for it (but my intention isn't to be rude...)
I downvote in the following situations:
OP is repeatedly not accepting CC and becoming combative- it's not a selfie sub, it's a makeup advice/discussion sub. If the OP wants to be butthurt, I'd rather she get extra butthurt over downvotes and not post a blurry selfie ever again.
Someone is giving objectively bad advice on technique, product, or they are obviously trying to be helpful but are wrong. For the OP to get conflicting advice, all equally upvoted, would be confusing. I think it's better that OP sees comments with good advice upvoted and bad advice sunk to the bottom. On the Internet, it isn't easy to tell if someone giving you advice has any credibility or not, so I think to help newer people out, it's better to downvote people who, to more experienced users, obviously are out of their depth. HOWEVER if they give valid advice but I don't agree with it (e.g. I think OP needs thicker brows, they think she needs thinner brows, but they give her valid advice for how to go about it, I wouldn't downvote, I'd just comment and offer an alternate opinion.) This is kind of a fine line, so it doesn't factor into my downvoting that much- I usually reserve it for unsafe advice ("just put some lemon juice on it!" or the like)
Compliments with literally nothing else (e.g. "OMG gorgeous!!!") If you want to compliment OP, say something specific that works and why, so that beginners can learn something instead of seeing a you-go-girl-slay-circlejerk and coming away with nothing constructive. You don't have to even add a single critique- you can say, "I love the way that shape of the liner elongates your eye," and that's more helpful and contributes something to the community. If it contributes to the conversation in some way, upvote. If not, downvote. That's how I roll.
If OP has posted before, I or someone else have provided super in-depth critique, they didn't reply, and deleted their post. I love discussing technique with people and I will spend hours doing stuff for strangers just because I like makeup and watching someone go from clueless > excited when they realize what they are capable is SUPER fun for me. I once made a tutorial for a single random MUA user because I wanted to help her with her bridal look, she was super sweet and receptive to CC, and I felt like with a few tweaks she could go really far. So with that context, it's fucking stupid when OP chirps that she wants CC and then flounces off upset when she doesn't get the compliments she thinks she deserves. There are a few regular posters on MUA who do this (and I know as I have been the giver of CC on multiple occasions, and for some I've observed it as a non-participant) and now I downvote their posts because they contribute to a culture/environment I'm not on board with. If MUA changed their rules so that CC was not allowed on all posts, I would leave because it would become a community that doesn't really suit my tastes or likes. I wouldn't just stay and downvote everyone because it doesn't go with what I like, but as a member of the current community, with its current rules, I do feel ok downvoting things I consider to be clutter and counterproductive, and for me, this counts.