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I guess most ppl just downvote on a whim and don't really care to give proper feedback coz they just specifically don't like some elements like gay shipping, this particular shipping, depicted theme or overall art quality.
possibilities why a picture is downvoted are endless actually, so don't try to analyze it, just draw what you like, and remember, success isn't about being the best, it's about always getting better.
if I had this situation, I'd probably put an effort, and draw a picture to show how much I don't care about downvotes :)