It isn't right for anyone to upvote content that is spam/low effort and make lots of money. If the content is a well thought out comment with depth to it that adds to the conversation and you upvote it so that it rises to the top and people see it, something like that is perfectly fine in most people's eyes. Sometimes when you comment on something your response goes to the bottom because other people's comments are upvoted and yours isn't. Upvoting yourself in that regard to get attention because you want people to see what you add to the conversation and you think it is important they see it.

Now if you are just making comments all the time and upvoting them(regardless of if the content is good or not) then you are draining the rewards pool on spam comments. There isn't an unlimited number of Steem payouts. The rewards pool is what pays us all, if lots of people upvote themselves for spam comments eventually so many people will be doing that and taking the rewards away from the entire community. There are times when the rewards pool of Steem goes down by a lot due to bad actors, those bad actors are then found out by the whales and the whales will downvote bad actors so they don't get any rewards ever again.