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You may be right about the self-voting thing. I upvote loads of people, including my own comments sometimes, but usually not. Anyway, I fell ill, for a day or to and couldn't participate much on Steemit, so I went and upvoted some of my older comments. It was a bit random, but I thought "Why not? Today I can't give it to anybody else.". Shortly after sadkitten appeared and down-voted a load of my comments. There was no explanation, no warning, no nothing. It could be that.

Yeah, it does appear so from your story.

Giveback to whom? You can only upvote content either you or someone else creates. Its our time an our investment. Why wasting eneregy on flagging?

Give back, as in upvote other people. Some accounts basically just upvote themselves and don't ever upvote others. As I said, I don't know much more, besides from what I've read about it quite a while ago.

I flag aswell, but I do that on plagiarised articles and clearly spam comments only. Why? Because I care about a good Steemit platform and that means keeping it clean of stolen content and spam.

Its philosophycal, flagging is altroistic. You invest your power for the sake of all. Egotistic beings take advantage of that. A good platform makes altroism redundat, since it balances and allow fair rewards. Incentives to give and limits on taking. That is the platform job. I am happy you are a good person, I hope you balance your power and give some to yourself. You deserve it.